Ah, the Gauntlet. Yeah, it's not a boss, but the function it serves is pretty close, if not the same. Can't really call it memorable though, all I remember is swimming through a bunch of purple passage-ways and shooting things. I remember thinking that the music for the area before the one the Creator is in was pretty fitting.
Jelly King comes pretty close to dead-last for me, but the annoying octopus takes that position. I agree with your list of honorable mentions, and I'm surprised no one's mentioned Simon yet. He wasn't my personal favorite, but he sure deserves to be honorably mentioned.
FYI: Mithalas is the city, Mithala is the god