Ahh my favorite enemy, it would have to be Vyrthur. The fight with him for the first time was amazing. Believe it or not I found it harder to defeat him than Harkon(But thats just me XDD)
Aside from that, as in like, general enemies (Bandits etc) I'd have to say I enjoy fighting Falmer, Warmongers especially, those bastards always manage to put up a good fight.