Halloween Sora is my favorite world form but I like most of them. Pirate Sora looks incredible, Monster Sora's design is interesting and lion Sora is cute.
The only one I actually dislike is his form in The Grid. It makes sense in that world and obviously resembles Space Paranoids Sora, but the latter was less ugly (and its colors changed when using drive forms, which was cool). For some reason Sora's KH2 face looked better than his KH3D face from KH1 in that kind of outfit.
But I find the world forms for Donald and Goofy much more interesting.
The main purpose for world forms is protecting world order, blending into the worlds by looking ordinary for each world's standard. Donald and Goofy mess up more often than not by turning into out of place hybrids that went wrong (Atlantica and Pride Lands) or freaking Duck and Dog pirates which would raise anyone's suspicion. Even monster Goofy looks too monstrous to be a Monstropolis monster.