Indalecio

I'd have to say the hardest boss in any game would have to be Indalecio from Star Ocean 2 with his limiter removed of course, with that limiter in place the guy is a push over. With his limiter removed he has around 2-4 million hp and can down your entire party in one attack regardless of level and gear. If thats not bad enough try removing his limiter in universal difficulty. I've heard he's impossible to beat in universal difficulty with no limiter.