To be honest i believe a DK has all the tools to be the best tank but really depends on the player's skills, at least for now that not everyone can kill stuff easily anyway.
early note: i'll be talking for the normal encounter but most stuff probably apply to hc too.
With the above been said, i believe (even though i havent really seen what the other tank classes can do) that DK on Halfus can be really good. As it seems the MS debuff is killing alot of ppl, but it can be managed in various different ways on our part but the raid also.
What can the raid do you ask? If your setup has paladins (ours most often has like 3 lolz) they can put hand of protection to the tanks which removes the MS stacks. Now you can either use it to reset them after a tank switch and allow healers to top your hit point, or during the actual tanking to just reset them and then cancel the aura.
Now on the DK part, what im basically doing is like, pop vampic blood early while tanking halfus so i can make the healers' life easier while they still can heal me due to no/low amount of MS stacks (plus DS/rune tap heal is always sweet with VB). While the fight progresses a bit and i get more stacks, depending what drakes you "use", i'll use bone shield or dancing rune weapon to lower the chance to get more stacks/mitigate some inc damage (avoidance on use trinkets help alot too) basically to survive until the tank switch/hop. Of course IBF and lichborne can be awesome if you get high on stacks as well.
Bottom line, avoidance is really great from what i see (especially on halfus), as long as hp isn't a problem and i really think it isn't thanks to crap load of defensive cds we got, and as i am a fan of avoidance im gonna follow that path.
Hope that helps even a tiny bit!
