Kingdom hearts 2 against Xemnas (when i found out that name was both Ansem and names with X added (like all the XIII's) i nearly exploded with plot-twisty glee)
the entire sequence because that son of a ***** would not DIE!
there were 5 stages i think to the entire thing, from fighting a dragon, to him in a a chair, to him again, to the final ultra-trippy battle in the eye-watering dimension that frakked with distance and your sense of decency. now that was hard enough to challenge, but easy enough to enjoy.
that said, Kingdom Hearts 2 had an entire room with portals to boss battles which i loved each and every one of, skill, speed and knowledge in every one, A+++ in my book of bosses. (or collections thereof)
hell, i just love kingdom hearts, the voices are a little annoying, but the story is well thought out, fun and as mentioned, that game has the greatest boss fights ever.