Hmm... let's see... Perfect Chaos in Sonic Adventure and Finalhazard in SA2 were hard, but only because the games were so full of bugs. lol. The final boss in Shining Force 2... I forget it's name, but it was hard. I'd say the hardest boss ever is King Gedol at the end of Sparkster (Mega Drive.) You think you've beaten him but then he switches places with you and it gets so tough and confusing! Great game though.