Ok, Cryx player so this is going to be interesting to read
Favourite: It's got to be Mortenebra. Sure she is simple but It's so much fun to get the assassination off. Mind you, I do feel bad when playing her against somebody who has never seen her in action. I feel I should be handing across her spell card and saying I'm not cheating, honest. I really can get boosted melee against all living, a free advance, ghostly with +2
ARM and re-rolls of attack and damage in one turn.
Least favourite: Either of the Goreshades. His fluff is good. He's trying so hard to save the elves but in game he just doesn't make it fun for me.