Even though mages probably can solo as well as a necro, my bet would be on a necro, as well. I find them to be the most interesting class in the game, and even more important, I'm really unmotivated for DCing (2-boxing) when I play my necro.
Mages aren't nearly as versatile as necroes, since they're all about damage, they don't have the more interesting parts such as FD (splitting!) and hp -> mana conversion.