5. Crawmerax
I'm still unable to kill this boss without using the underhanded tactic of hiding in a glitched part of the map where he is unable to target you. Even then, it takes an incredibly long time and can still be pretty treacherous. Like the Ruby and Emerald Weapon in Final Fantasy VII, I'll never feel as though I've truly completed the game until I've killed this guy the proper way. That said, it still feels good every time he explodes into a plethora of orange and purple items that take as long as the fight itself to sift through.