The most memorable for me would be Naxxramas, I think. Can't tell exactly why though, perhaps because I kind of enjoyed the whole setting of it and the atmosphere there.
I've actually been doing some "historical" gaming posts lately, covering noteworthy games released in certain years (just made two about 98 and 2000) and WoW is one of the first MMO kind of games that required really smooth teamwork from players, of course many other games adopted this concept after