My vote goes to Mieaevar. Ran into this fellow in the unreal caves and got soundly thrashed. He's got all the Balor/Greater Balor skills (corrupting melee attack, regenerates, teleports, summons monsters, etc) but also casts a variety of nasty spells such as 200+ damage energy blasts, death ray (I think, I had resistance), etc.

What's annoying about him is you need to stay out of a direct line to avoid the energy blasts and death rays, which means you most likely have to fight him in a room. And in a room there's ample space for him to summon monsters, so you can't get a good shot at him with slaying ammo. And if you do manage to hit him, he's got tons of hp and will quickly regenerate while you're busy clearing out his summons. Bah.

Which is your candidate as the most annoying boss monster?