A (usually) accurate and up-to-date style list can be found here at CoC: Reaver Styles

This page is for analysis and notes on the styles - namely those in the Flexible line.


The Style Chains:

Block Chain:
Viper - Has a 5-second stun. Depending on your swing speed, you can get one or two more styles off before the stun wears off. One strategy is to move behind your target and Leviathan while they're stunned.
Copperhead - Has a "target is slowed" effect, but that is rarely noticeable (not sure it even works).
Taipan - No special effects.
Cobra - Has a very nice lifetap effect. The lifetap does about 300 damage against a yellow with neutral resists and returns 100% of the damage done in health. The damage also increases along with WeaponSkill.

Pre-50, the block chain is especially useful if you need healing. You can use the stun to perform an Indigosnake or two, or (if you're high enough to have Cobra), do the whole chain and get a Lifetap off.

Post-50, the chain is best used after the Parry chain, in order to make the most of the Parry chain's better stun.

Parry Chain:
Kingsnake - Has a bleed effect, making it a dangerous style to use in parts of Trials of Atlantis.
Tigersnake - Nothing very special about it...
Anaconda - 9 second stun.

Pre-50, the Parry chain is useful mostly for damage, to get a few Indigosnakes off during the stun effect of Anaconda.

Post-50, the Parry chain is incredibly useful for the chance to get multiple (as many as 4 or 5, depending on your swing speed) Leviathans off. It is best used before the block chain, so that the 9-second stun isn't cut in half.

Side Positional:
Indigosnake - A very, very nice style at low levels, because the Lifetap effect does a lot of damage and, of course, heals you. It's unarguably the best style until level 35 or so. At later levels, though, it is often resisted and, because the proc damage doesn't scale with level, it doesn't even do a great deal of damage.
Constrictor - Has a hindered effect, which is of limited value, since if you're in melee range you probably don't really need to hinder a target.

The side positional chain is rarely useful, either pre or post 50 - only in early to mid levels when Indigosnake is still good on its own and your target turns to face you after Indigo hits.

Other:

Python:
Presumably Python was intended to be the Flexible line's primary anytime style, but in fact it does more harm than good. The endurance cost is about 1/5 of your total end, and the Strength debuff on it is AE and breaks mezz. The cost (endurance, risk of aggro or mez breaking) isn't worth the small benefit unless you are in a group with a Paladin, can't use positionals, and crowd control isn't an issue...

Boomslang:
In the absence of another useful anytime style, the detaunt becomes the top choice. It doesn't add any damage, but the high defense bonus means you'll have an increased chance to get off a block or parry chain after using it.

Leviathan:
One of the best styles in the game and, arguably, the best. The proc does anywhere from 200-400 damage, depending on the target's resists (Leviathan is cold-based, unlike every other Reaver style and spell) and your WeaponSkill. And though it is rear positional, it is easier to pull off than other styles with similar damage potential.  Generally, whenever you have a chance you should use Leviathan, even if it means cycling between mobs or otherwise altering your strategy. It is best used in conjunction with Boomslang, because the damage will accrue a lot of aggro (and if you have aggro, you can't use Leviathan). Another (fairly obvious) Leviathan strategy is to team up with a Paladin, who should have little trouble getting and keeping aggro, so you can stay behind the target and Leviathan to your heart's content.

Asp:
One of the more unique styles in the game, Asp has an increased attack range, whether used from Behind (the intended position) or not. It is useful when chasing after an enemy or, very rarely, when you can't get into normal melee range - they are on the other side of one of those little window slits in a keep, for example.