Protection specced Warriors with appropriate gear are the best Boss Encounter tanks in the game. They have the best damage mitigation vs. single targets, and the largest and most versatile array of tools for tanking, which includes threat generating tools such as Revenge, Shield Slam and multiple taunts, many mitigation abilities, and finally a few panic buttons like Shield Wall and Last Stand. The only problem area for warrior tanks is aggro generation, particularly their initial aggro generation is lower. In long fights, all three main tank types actually create comparable amounts of aggro. Finally, Warriors depend more on their gear than any other class (this can also be a boon, because a warrior can increase his abilities further than the other classes with better gear).
http://www.wowwiki.com/Tank
Let me read more about the DK. Im not so sure about agro Ill be killing things kinda fasts anyways. For pvp warrior has the best damage mitigation also I assume.