IMHO, there is no "best" mitigation stat to stack. It's a balance. I usually insure I have enough Stam to survive back to back hits from what ever boss I'm fighting, I try to keep my avoidance ~45% (not block, parry and dodge). Expertise AT LEAST to soft cap; I rarely ever have more then 3% hit, I muc prefer expertise, even to hard cap, then hit. (Expertise is always better then hit.)
I personally gem for Stam in blue sockets, def/stam in yellows, expertise/stam in reds.
I do not believe there is DR on block, but block is very underwhelming against most bosses.. the reality is block is gravy, and you should never gear for it unless your building a trash set.
This is my personal strategy, not everyone agree. Some people stack stam, others avoidance... it's my personal belief that a balance is required. Further I consider it very important to analyze each encounters mechanics and tailor your gear for that boss. For example, tanking Steelbreaker at Iron Council is a very different gear set-up then tanking Auriaya. Steelbreaker hits slow and hard, and if your dispeller is slow... high EH is a must; however Auriaya hits much weaker and quicker.. and avoidance / shield block can really increase your healers mana efficiency.
I hope that helps answer your questions.