yep, light sorcs make the best key runners. MF% doesn't matter for keys, so just load up on the +skill gear. (although MFing Nihlatek isn't a terrible idea) The only one that can farm D keys faster is a hammerdin with enigma.
A strong fire based sorc (meteorb or blizzballer with lots of fireball synergy & fire master) can do ok too. But the cold/fire sorcs need a merc to kill the countess (she's always both fire and cold immune). This may sound like a big disadvantage, but my meteorb merc takes her down in like 2-3 pokes. You can always static her and bump her with telekenisis to help as well. The summoner drops easy to just about anyone. Nihlatek is a pain (make sure you have a natures peace). He constantly spawns cold immunes, so a stong fire build is needed. You still have to watch out as there's often fire immunes in his chamber too.
ps: Or you could go fury / feral ww. I've found that running D keys & T Keys to be most satisfying with my wolf (lots of r/w). He's pretty much immune to everything Nil / Countess can throw at him, but the wolf handles the arcane sanctuary very poorly (no tele while in fur).