I also have another monster suggestion: make bile demons joke monsters.
Oh, and here's another all-time favorite arch-nemesis: Greater Draconic Qs. Personally, I think they're worse than Master Qs, since they
always summon high dragons, wcich is just about the most powerful summon there is.
Another one: Pit Fiends. Demons that summon ancient dragons. Ouch.
And another: chaos hounds. God, I hate those stupid fleabags. Almost all my best Lost Soul chars have been killed by those guys...
They and the Horned Reapers are enemies that have only arrived (relatively) recently in the band universe. In any case they're newer than ZAngband.
There's another arch-nemesis of mine: the infamous Horned Reaper. Huge number of HPs, *insane* melee attacks, and that awful summon spell...
All these guys pale in comparison to Great Wyrms of Balance, however. I personally think they're even worse than GWoPs, since they summon more often. They also seem to move faster than any other kind of dragon, though that's probably a bug in r_info.txt.