Rabid Skags (good Exp though)... have yet to meet a Rabid Stalker but I expect they are just as bad.
Cursed Pirates on the DLC. I was in Co-op last night and they proved to be a total pain. I was a Lv 25 Gunzerker and the other player was a Lv 25 Assassin. Best results seemed to be with my (amazingly Lv 7) Wild Kerblaster.
Tinkles (from the DLC) on TVHM proved to be something of a pain and I eventually resorted to using the Skiff weapons.
And finally... those seriously strong Armoured Maniacs.
Probably Rabid Stalkers except for when playing as Axton. Sabre Turret >>>>> Rabid Stalkers
I'd say Goliath Blasters are right up there with the Rabid Stalkers though. If you don't know your dealing with a Blaster when he shows up or kill him ASAP you pretty much are guaranteed to get downed(without using Sham).
Foolish mortals and your petty squabbles Giga-Graviton > Zanmato