I'll put the sun form as my favorite, because crawling through the Abyss with just a little headlight wondering what you might run into in the dark was rather fun, and tense (I rarely used the charge-up flash power, to keep the dark feeling). Of course by the end of the game I'd been through there enough times to be quite familiar with everything down there, but the first time through was the best part of the game to me.
I also liked the nature form a lot. It might not be as practical as the energy form but it was fun to try to do things like stick yourself in a little cactus tent and just wait as the enemies knock themselves against it, or throw it in exactly the right spot to take them out in one hit. Plus its a quick lazy way to harvest a bunch of fish before you get the dual form, if they're close enough to a wall. It's also the coolest-looking I thought.
Third favorite is beast form. I didn't use the shots much because they're hard to aim, the way they sit still unless you're moving, sometimes you don't want to be moving towards the enemy to be able to shoot towards them (but I guess in those cases you shouldn't be in beast form.

), but I liked it for the speed and ramming through a bunch of enemies. And for the way it fits into the storyline, she said something about how she's worried about how much she's starting to enjoy the destruction (or something like that?) sometime before, and then you're dashing into things and devouring them left and right. Kinda creepy, especially right after hearing Mithala's story.