One has Holy Freeze and one has Red Hose Lightning. With melee characters I have a harder time against Duriel, though I'm pretty bad with melee. With Ranged and spellcasting characters I have a harder time against Diablo, and I'm much better with these characters.
I've only got two characters, a Barbarian and a Necromancer. With the Necromancer I only got up to act three so I didn't face Diablo but I thought Duriel was a lot easier because he was distracted by my minions whilst I fired Bone Spear. Shame because I still thought he was a pain in the a*se.
Griswold = hardest boss. Between those 2 though, Duriel. For your likely state of preparation before that fight, the hardest. Clearly Diablo is harder to kill, but you should be more prepared for that one. Unless you suck. Which is entirely possible. Hey dark, long time no see.