Favorite:
Kuuga Ultimate - Most everyone seems to have put this on their lists as well, so....
Kiva Emperor - Kiva's base form was okay at best, but honestly, the enlarged helmet crest and cape look really cool. Maybe I'm just a sucker for capes.
W CycloneJoker Xtreme - W's designs are amazing (bar Accel's base form), and XTREEEEEEME's giant crystal-ey center and X-shaped helmet are the apex of that. Better than Gold-Extreme.
Least Favorite:
Den-O Liner Form - Most people don't seem to like Den-O's forms. I don't agree, but even if you don't agree, most of them are at least interesting. Liner Form isn't. I understand the thematic importance behind it, but that doesn't mean it isn't just a dull, dull form with an ugly sword that the suit actor can barely hold.
OOO PuToTyra Form - Giant wings on his head. Giant tail that can grow and shrink. Horn-claw things on his sholders that can extend out for his stupid, stupid finisher. I normally like purple, but I really really hate almost all of OOO's forms. Especially this one. And TaToBa.