Blood because its not much slower than Frost for killing mobs and its a lot more survivable if it starts pulling too many.
Also pvpers hate taking my blood dk on as they can tank them for ages, so many times I've checked on my blood dks and seen them next to a broken corpse of some guy who figured he could take my bot out.
If I was on a pve realm I might roll Frost but to be honest Blood is just so good you hardly ever have downtime.