It kind of depends on the fight and what you are tanking. Things that hit fast and for not as much, wars/pallies are typically better. For things that hit harder DKs/Druids are better. DKs were OP as hell in 3.1 and they were pretty much just beaten up with the nerf bat in 3.2, so DK tanks have been seen less frequently but they are by no means terrible tanks.
I'll let someone that knows way more about DKs than I do give more in depth explanations though.
Edit: oh and block doesn't really matter THAT much.
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