My particular favorite warcaster is Kreoss- both versions are good, though I particularly like the original version best.
Gamewise, he's a solid, all-around well balanced warcaster who can do just about anything I need him to.
Fluffwise, he stands for justice, strength, and force of will- he's not as hot-headed as some of the other members of the Protectorate, but he's willing to lay down his life for his beliefs.