Dwarves and Dark Elves for me (and Undead). but I guess it's more of necessity as I bought the game with two friends and we've since divided it by factions (with me playing the aforementioned ones).
I'm more of a casual player, so I like to mix my decks up from time to time and try to have as many decks available at all times. Recently made a dwarf rush deck, based on the new Grudge cards. Probably won't work most of the time, but really fun when it does.
My favorite has to be my Dark Elf Control deck nicknamed My Little Pony since it's so cruel it makes your opponent go into denial. Also it has pink sleeves.