Reavers get the proverbial short end of the stick when it comes to Artifacts. Only one "offensive buff" effect may be on a character at a time; the Reaver Lifetap proc buff counts as one, and as a result many Artifact abilities do not stack with it. Since the LT proc buff (in conjunction with the LT shout) has been used in the past as a reason for the Reaver not to have other abilities or spells to help with defense, it could be argued that a Reaver without the LT buff available is underpowered.

Despite these shortcomings, Artifacts are from useless. Artifacts can add abilities you're not likely to get elsewhere, albeit at a high cost in time. Here are the Artifacts which are probably the best for each slot:

Chest
Guard of Valor
Level 0: 40 Hits, 18 Strength
Level 1: +4 to Strength bonus cap
Level 2: 15 Dexterity
Level 4: +4 to Dexterity bonus cap
Level 5: Magical Ability - Shield Debuff proc.

Lowers the target's Armor Factor (AF), resulting in worse protection against some forms of attack.
Value: 50
Duration: 1 minute
Damage: Body
Spell has a chance of casting when this armor is hit by an enemy.

Level 6: 4% bonus to melee damage, 4% bonus to magic damage
Level 8: 4% bonus to style damage, 4% bonus to spell range
Level 10: Magical Ability - Damage-over-time

Inflicts damage repeatedly to the target over a given time period.
Damage per tick: 40
Duration: 30 seconds
Frequency: 5 seconds
Damage type: Body
Spell has a chance of casting when this armor is hit by an enemy.

A 4% bonus to four different bonuses make the Guard of Valor one of the best Artifacts, period (at least for a Reaver; other classes don't all get the same bonuses). The abilities, both reactives, allow you to use the armor without having to worry about /use-ing it. The actual stats on the hauberk are rather pathetic, but the bonuses more than make up for that.


Legs
Wings Dive
Level 0: 6% Cold/Heat
Level 2: 6% Spirit
Level 4: +40 to hit points bonus cap
Level 5: Magical Ability - Cushion of Air. Reduces falling damage. Five minute duration.
Level 6: 12 AF
Level 8: 6 Fatigue
Level 10: Magical Ability - Aura of Life.

Heal over time
Target regenerates health during the duration of the spell.
Value: 30
Target: Group
Range: 500
Duration: 20 secs
Frequency: 5 secs

The stats are only decent, as are the special abilities. However, it's fairly easy to get (can be done with 3-4 people or any class + a Necromancer) and to level (Inishail Island is never camped and has several large camps of yellows/oranges). Unless you've got some extremely good jewelry items, however, you'll probably be better off using some player-crafted leggings.

Arms
Arms of the Winds
Level 0: 18 Dexterity, 12 AF
Level 1: Fatigue 6
Level 2: Magical Ability: Disoriented

Target is disoriented. They have less chance of landing melee attacks, and spells have a greater chance of affecting them.
Melee-to-hit penalty: 5%
Resist piercing penalty: 5%
Target: Targeted
Range: 350
Duration: 1.0 min
Casting Time: Instant
Damage: Body
Spell has a chance of casting when this armor is hit by an enemy.

Level 4: 7% crush resist
Level 6: 7% slash
Level 8: 7% Thrust
Level 10: Secondary Magical Ability: Damage over time / disorient

Inflicts damage to the target repeatedly over a given time period. Also disorients the target, making it more difficult for them to strike at melee targets.
Damage per tick: 35
Target: Targeted
Duration: 30 sec
Frequency: 5.0 sec
Casting Time: Instant
Radius: 700
Damage: Body
Spell has a chance of casting when this armor is hit by an enemy.

The stats are only marginal - the physical resists are handy, and Fatigue is always good, but there aren't a whole lot of bonuses. The special abilities are useful for those with multiple Artifacts or charged items, because they're always on and don't face the usual problems with the 'use item' timer. There's a lot to be said for passive abilities, and these should proc fairly often, since the arms are an oft-hit piece of armor. The sleeves are relatively easy to get, assuming you can find a Necromancer to help, since there's only one mob to be fought.

The Arms of the Winds are the best available for the Reaver primarily because they're the ONLY Artifact sleeves available.

Helm
Crown of Zahur
Level 0: 18 Constitution
Level 1:  8 fatigue
Level 3:  5% crush
Level 4:  5% slash
Level 5: Magical Ability - Efficient Endurance

A portion of all heals cast on the target restore endurance instead.
Bonus: 10%
Target: Self
Duration: 10 minutes

Level 7: 5% Thrust
Level 9: +5 to Constitution bonus cap
Level 10: Magical Ability - Makes heals cast on you 10% more effective. Unknown duration.

Low on utility as most Artifacts are, but the magical abilities are very nice. Consider the level 5 ability - 10% of a normal heal is from 20-40. Aside from Fatigue bonuses, players have 100 endurance - so that's a 20%-40% endurance heal in addition to the 90% of the heal going to health. Apparently it does not work on the heals from lifetaps.

Gauntlets
Maddening Scalars
Level 0: 40 Hits
Level 2: 15 Constitution
Level 4: 15 Quickness
Level 5: Magical Ability: Shield/damage return.

Blocks a portion of melee damage dealt to target and damages the attacker instead.
Value: 40%
Target: Self
Duration: 10 sec
Casting time: instant
Spell has a chance of casting when this armor is hit by an enemy.

Level 6: 3% bonus to melee combat speed
Level 7: 3% bonus to melee damage
Level 8: +5 to Constitution bonus cap
Level 9: +5 to Quickness bonus cap
Level 10: Magical Ability - Offensive proc. Also has a shapechange effect which turns the user into a wolf.

Frothing Wound
Inflicts damage to the target repeatedly over a given time period. Also inflicts a disease on the target.
Damage per tick: 20
Target: Targetted
Duration: 1 minute
Frequency: 6 seconds
Damage: Body

A 3% bonus to combat speed and damage are the best part of this Artifact. The level 10 ability does not stack with the Reaver LT proc, so won't see much use. The level 5 ability, though, being reactive, could be extremely useful, even if it is very short-lived and on gloves. Constitution and quickness bonuses and cap increases seem almost tailored for a Flexible Reaver - attacking faster does more than anything else to increase damage, and our hit points are one of our great weaknesses.

Boots
Just go ahead and get crafted; having an Artifact here is a waste of your time.

Weapon
Flex
Snakecharmer's Whip
16.5 DPS, 3.5 SPD
Level 0: 3 Flexible
Level 1: 4% Heat
Level 2: 4% Cold
Level 3: 4% Energy
Level 4: 3% bonus to melee combat speed
Level 5: Life-/End-/Power-tap proc. Of proc damage done, returns 50% in health, 30% in power, 20% in endurance. Has a chance of casting when this weapon strikes an enemy.
Level 6: 4% Matter
Level 7: 4% Body
Level 8: 4% Spirit
Level 9: 4% bonus to melee damage
Level 10: Magical Ability - Shield/Damage Return. Blocks a portion of melee damage dealt to target and damages the attacker instead. Value 20%, 5 minute duration, Target Self.

Now that the SCW is 3.5 speed rather than 4.0, it is probably the best whip for a Reaver. A 4% bonus to melee damage (with no mob type restriction) somewhat makes up for the speed lost compared to a 3.0 crafted, and it is possible to get the weapon VERY close to cap. For those who don't care to rely on Leviathan for the vast majority of their damage, the proc and higher melee damage per hit more than make up for the slight slowness.

Weapon
Slash, Thrust
Crocodile's Tooth Dagger
Level 0: 27 Strength, 27 Dexterity
Level 2: 2% bonus to melee combat speed
Level 4: Proc which takes 40 Strength/Dexterity from your opponent and buffs you by the same amount.
Level 6: 3% bonus to melee style damage
Level 8: 4% bonus to melee damage
Level 10: Magical Ability - Summon Item. Creates a dagger in the user's inventory with the following stats:

16.5 DPS, 3.5 SPD
4 Slash
10% Spirit
36 Hits
+10 to Strength bonus cap
+10 to Dexterity bonus cap

Magical ability: Damage over time/disease proc
Inflicts damage to the taget repeatedly over a given time period. Also inflicts disease on the target.
Damage per tick: 25
Duration: 40 sec
Frequency 5.0 sec
Damage: Matter

Secondary magical ability: Damage Shield
Damage 4.2
This spell is active while the item is worn

The dagger comes in two different varieties, and so can be used by those using slashing as well as those using thrusting. The level 10 ability is more or less useless, but bonuses to style damage, melee damage, and melee speed are very nice. The level 4 magical ability may not be especially useful for the buff part, but the debuff is another matter.

Weapon
Crush
Bruiser
Level 0: 40 Hits, 15 Strength, 15 Constitution
Level 2: 6% Spirit
Level 4: 6% Energy
Level 5: Realm Lore. Displays name, guild, race, class, spec, stats, resists, buffs, and realm abilities of an enemy realm player.
Level 6: 8 AF
Level 8: 4% bonus to melee combat speed
Level 10: 4 Fatigue. Magical Ability - Siege Lore. Target does more damage to keep doors and siege weapons. Bonus multiplier: 15. Duration 90 seconds.

It has more bonuses to stats than your average Artifact, and the level 10 Ability makes you pretty much a ram every 15 minutes.

Shield
Cyclop's Eye
Level 0: 40 Hits, 5% Heat
Level 1: 5% Matter
Level 2: 5% Body
Level 3: 5% Spirit
Level 4: 5% Energy
Level 5: Stealth Lore

Target has a greater chance of uncovering stealthed opponents. If the target has the stealth skill, their ability to use stealth is greater as well.
Stealth bonus: 35.
60 second duration.

Level 6: 8 AF
Level 8: +15 to hit points cap
Level 10: Realm Lore - Target an enemy and use, and a window will pop up telling you their name, guild, race, class, spec, stats, resists, buffs, and realm abilities.

Well, Mythic did it again - they overnerfed an Artifact. Stacking of Stealth Lore bonuses was overpowered, making life as an assassin nearly impossible; but rather than eliminating stacking of SL, Mythic reduced the duration from 10 minutes to 1. While you still might get lucky and find a stealther during that 1 minute it is up, the ability is a shadow of what it once was.

The Cyclop's Eye still has good stats, though, with a utility value of 60 in addition to 8 AF and a HP cap increase, so if you can't get one of the great shield drops from a ML encounter, it's worth having.

Cloak
Shades of Mist
Level 0: Stealth 3 (stealth classes) or Parry 3 (melee classes)
Level 1: 15 Strength
Level 3: 15 Quickness
Level 5: Magical Ability - Stealth Lore. 

Target has a greater chance of uncovering stealthed opponents. If the target has the stealth skill, their ability to use stealth is greater as well.
Stealth bonus: 35. 
1  minute duration.

Level 6: 10 AF
Level 9: 5% bonus to melee combat speed
Level 10: 4 Fatigue. Magical Ability - Defensive proc buff

Shade Skin
Target:  Self
Duration  10 min
Casting time:  2 sec
Sub-spell:  Adds a shield to the recipient which will temporarily absorb some of the damage type specified.
Absorption:  100%
Value:  200
Target:  Self
Duration:  30 sec
Casting time:  instant

The Harpy's Feathers Cloak used to occupy this spot, but 1.68 nerfed it in such a way that it lost an estimated 11.3 in utility value. Shades of Mist has the same haste bonus and more useful special abilities - unlike the proc buff of the Harpy's Feathers, which is offensive, that on the Shades of Mist is defensive and stacks with the Reaver LT buff; Stealth Lore, while hardly as useful as it used to be, is nice to have anyway.

Necklace
Goddess' Necklace
Level 0: 3% Body/Matter/Spirit
Level 1: 9 Strength
Level 3: 9 Dexterity
Level 5: 9 Constitution, Magical Ability - Heal. Self-targeted instant heal with 150 delve value.
Level 7: +10 to AF bonus cap
Level 10: Magical Ability - Stealth Lore.

Target has a greater chance of uncovering stealthed opponents. If the target has the stealth skill, their ability to use stealth is greater as well.
Stealth bonus: 35.
60 second duration.

The Goddess' Necklace is the only Artifact necklace available to the Reaver. The stats are notably weaker than those of the most popular item in the game, the SI quest necklaces (Ancient Copper Necklace, for Albion)... and with the 1.68c nerf to Stealth Lore, it probably is not worth using.

Gem
Dream Sphere
Level 0: 5 Fatigue, 15 AF
Level 2: 40 Hits
Level 4: +40 to hit point bonus cap
Level 6: Illusion. Turns you into a black lion. There is a low-damage DoT associated with the form. The shapechange lasts for 10 minutes.
Level 8: 5% Matter/Body/Spirit
Level 10: Illusion. Changes user's group into a black lion. User gets a +10% chance to hit in melee and a low-damage DoT; group only gets the shapechange. Lasts 10 minutes, useable every 15. Breaks when you are attacked - negatively affected in any way.

Despite the near-total nerfing of the level 10 ability (which makes it essentially useless for a Reaver; you'd have to remove your LT proc buff for a 10% increase to chance to hit, which will almost certainly disappear as soon as fighting starts), the Dream Sphere isn't bad. Fatigue is remarkably useful, resists are always good, more AF is nice. Leveling the Dream Sphere is easier than most Artifacts, unless Barrows is severely overcamped. Even so, a few dozen hours leveling an Artifact with useless special abilities and only average stats is probably a waste of time.

Ring
The Ring Artifacts available to the Reaver (Band of Stars, Ring of Dances, Scorpion's Tail Ring) are poor fits for the class. You should probably stick with a quest reward (Cenalon's and the Ring of the Seven Storms are prime candidates), or a good RoG from ToA.

Bracer
Snatcher
Level 0: +40 to HP bonus cap, +5 to Constitution and Dexterity bonus caps
Level 2: 4% Crush
Level 4: 4% Slash
Level 5: 7% bonus to all bounty points earned
Level 6: 4% Thrust
Level 8: 8 AF
Level 10: 4 Fatigue, 6% bonus to all realm points earned

The stats are decidedly anemic, but the special abilities are passive and unique. A 6% bonus to realm points earned is... well, uber comes to mind. Other ToA bracers have extremely high utility, so you may have to give something good up to get those extra RPs - but especially if you get them early in your RvR career, they're worth it.