Daemons, Vamps, DE and WoC will be hard.
Daemons: Let's face it, they can be really overpowered.
Vamps: All those massive units just keep coming back and auto-break you.
DE: Higher WS, Eternal Hatred, insane magic items/ banners= dead lizzies
WoC: Strong elite force who also have cheap but pretty nice marauders(who are in my opinion underpriced(4pts base, I thought))