Paladins have been the solo instance kings for a while now, from BRD to Onyxia to Blood Furnace, but there may be a new contender in town. Felblood let us know that his level 80 Death Knight (I believe this is his Armory page) has soloed one of my favorite instances, Zul'Gurub, going from the snake boss all the way to Hakkar with just his character.
The snake boss, he says, was easy, just a nuke. The bat boss Jeklik silences, and a few of his resists failed on her, so she got some healing off, but she still dropped. Panther and spider went down all right, though the spider's webbing apparently kept him from healing as well -- being silenced, he couldn't cast disease, so Death Strike didn't heal. Bloodlord Mandokir was super tough, apparently -- I can imagine that watching would be pretty nuts during that. On Thekal, the tiger boss, his problem was that he was killing too fast: the boss would get ressed when one of the adds died early. Eventually he just brought them down to 50% and then just nuked all out, and the second phase was easy.
And Hakkar was anticlimactic -- he just basically tanked and spanked. He tried to anti-magic the Blood Syphons, but they didn't heal for much, so he just wailed on Hakkar until he was the last one standing. Quite a feat. Of course, at level 80, he was 20 levels above where this 20 man instance was meant to be, so we probably haven't seen the end of the level 60 content being soloed. But it seems that Death Knights are almost more equipped than Paladins to bring down some of the raid content on their own.