Yup, Duriel was a tough one. My main character was a paladin and I had a pretty hard time getting him down. But still nothing beats Diablo in part 1
When you had slain your way through the succubus hordes and finally meet Diablo it was really great to fight him, even if you died several times. I really loved the 'decend to hell' theme of Diablo 1. Hope they bring that to the third part.