Diablo 1 was better than 2 in my opinion
Especially after installing it's expansion (Hellfire). You just have to love the new items (especially the staff of the jester and the magic traps/bombs). Na'krul was too damn easy to beat. I had a harder time when I first entered the crypts. The defiler was hard though...
As for Diablo 1 classic, my favorite bosses are the Warlord of Blood because of it's voice and Archbishop Lazarus because of his overall coolness