Well, I wouldn't call it the "best" but actually about any DK I met during the first few months of WOTLK killed my poor Warlock for good...
Other than that... Lately I've played a WSG on my Moonkin. I've got decent 'survival' Equip (1.4k resi, Frost Badge Tank Trinket), most enemies need quiet some time to kill me. At least I thought so, until I met a organised group from another server. They had pretty much BiS PvE gear (well, at least the hunter I looked up had). The hunter alone killed me ridiculously fast. I tried it several times to catch one of these guys alone. I caught the hunter twice, but both times he killed me through Barkskin very fast.. I couldn't help but laugh ^^