LMFAO, poor Gun Slinger.
Anyways, my favorite is Royal Guard. I don't know why, but there is something about canceling for a perfect guard in mid-move that is just Godly IMO. Plus Royal Release is almost disgusting as to how much damage it can do.
Next would have to be Sword Master. I just love the variety of moves that you gain with it to add for great combos.
Trickster is next. So simple, so easy to use, yet so very useful.
Next is Dark Slayer. It is so simple, yet so damn powerful. Unfortunately, I dislike how simple it is as a style. I wish it was a little more like Vergil's Dark Slayer in DMC3 (which is kind of like a different version of Trickster), yet incorporate Yamato to kind of have the best of both worlds type.
Finally, it is poor Gun Slinger. Simply put, while it has some nice moves, it completely fails as a style in comparison to all of the other styles. However, with being able to change styles in mid-combo, it does have its uses.