i'm not exactly sure, but the easiest has to be:
easy: beserker lord
hard: the bryyo seed boss, the statue thing (most of the big bosses i didn't beat on first try, had to figure out exactly how to beat them XD)
coolest: omega ridley definitely
by the way, i haven't beaten the game yet, i'm only to where i'm just about to board the battleship orbiting the pirate homeworld, so this is subject to change.
Thanks to ps2man617 for awesome sig and avy(name put on avy by MV).
Been a while since I was on supercheats, so my profile is mostly way outdated
Dranixger =
Dragon + Phoe
nix + Ti
ger