Anti Form was awesome... I so wish that they would make it permanent. Funny story, a lot of the time in boss battles I'd go Anti Form instead of whatever form I selected, and I'd be doing better in Anti Form.
For instance, once when I was fighting Xaldin, and I selected Master Form (I'd already gone Master Form once before, with little improvement), but I went Anti Form instead. At first I thought... "Oh, geez... I'm screwed"... but I was doing better in Anti Form than I had done in Master Form.
One question though... why didn't Yen Sid warn Sora about Anti Form? He should have known about it, right? Other than Anti Form, my favorite is Final Form, but I prefer to use Valor Form.