I've honestly have had a harder time fighting some random encounters then bosses. The bosses usually have a routine they do, know the routine and any boss is simple.

I don't remember the name, it's been awhile but the 2nd to last boss in The Void in FFV, (right after Shinryu but before Ex Death) that boss is my most hated boss.

Red Dragons are the main problem I have in FFVIII. (without Odin/Gilgamesh)