Heyyyyyy, Kingdom Hearts is one of my favorite games. I'll vote for Hollow Bastion in KH2 because it had so many things that I liked in it. I always looked forward to the 1000 heartless battle. Later in the game, you can fight Sephiroth and he is one of my favorite bosses.
ill go with end of the world. hollow bastion was just too confusing and annoying for me lol. but hollow bastion 2 was kind of boring if i remember correctly.
I don't know a thing about any of these places, since they're all from a series of games I've never once played... but that first Hollow Bastion looks like a spectacular piece of architecture, so I shall cast my vote for that one! =)