Well, hmm... hard to pick one. My favourite is probably Antiform, though, just for the sheer fact of how different and stark it is from the rest. I also enjoy ending a normal battle while still in Antiform... this'll sound really wierd, but I think Sora looks so cute sitting there all dark, misty, and feral-esque hopping around and glancing about. There again, I also think Shadow Heartless are adorable.
As for which one I enjoy playing the most... Final Form would have to be my choice. Antiform, as cool as awesome as it is, has too many ramifications to enjoy it during boss battles (WHY does it always happen during Xaldin?!)... Final Form is usually an I Win button in most cases, or it'll at least turn the tide of battle in a tight spot.
However, I usually abuse each drive form depending on when I get it.
Valor Form is usually maxxed for it's possible level by the time I enter Olympus Coliseum. I generally don't bother much with Wisdom Form, although I'll level it up (on Proud Mode, anyway) before I begin the whole script for Hollow Bastion (the second visit, when you'll enter Space Paranoids) because I find it comes in handy for battles beyond that... plus it's easy to level up when things are so much younger than you. Then once I get Master Form I abuse that like it's going out of style... it's a very handy form for killing groups of enemies. And then, of course, once I get Final Form I abuse it like crazy and kind of kiss Valor and Wisdom Forms goodbye. In fact, once I get Final Form, it's very rare for me to use any of the other forms because Sora's basic moves are so much more advanced than those forms. But it's hard to argue with a duel-wielding silver glowy boy floating through everything and tossing lightning around with just about every strike lol.
I love Final Form with Fenrir equipped and maximum Negative Combo's equipped. Something about going into a group of Heartless and just obliterating them all by pressing X once... priceless.
But, yeah, overall Antiform is my favourite just based on look and coolness factor. Plus Sora's so cute when he's all evil! =p