Flametongue: Benefits from +spell damage, so is usually the choice of Elemental. Good against high-armor targets since it's magical damage.
Windfury: Used to be the be-all end-all of weapon enchants... until it got nerfed to the ground. Still powerful, but only if you have the equipment (enhancement) to back it up.
Frostband: Similar to flametongue, but with lower damage to compensate for the proc. Selectively useful for PvP (to gain/keep distance) or PvE (for runners), but randomness of proc can get frustrating. Not good if you're just looking for damage.
Rockbiter: Almost useless. This used to be our tanking enchant (it used to add threat), but now is only good if you're under-geared and don't get as much benefit from FT or WF.