...in my humble opinion.... Protection Paladin. I got Kruul on every other tank, but I just can't seem to DPS things down fast enough before I end up with like 3 Infernals and everything goes to shit. I think I'm just doing something wrong because I'm like 20 ilevels higher than when I did it on my DK, DH and Druid.
So even if you haven't tried them all, what is the hardest one so far for you?