Don't be silly. Ulduar is regarded as the best raid instance in WoW. It had very diverse bosses; fun mechanics and a lot of lore.
Icecrown is regarded as the second best raid I guess, making WotLK top contender for best expansion from a raiding PoV.
Argent Tournament raid was bad. Naxx wasn't really bad IMO, people just hate on it because it was rehashed content (ironically by popular demand as very few, including me, never got to experience it in Vanilla).
That said. WotLK was a superb expansion when it comes to overall experience (zones / quests / lore / raids). But the poll specifically mentions: " endgame world content". That doesn't say "endgame content" but "endgame WORLD content" which to me means open world things to do at max level. For WotLK that pretty much meant Stormpeak giant dailies; Icecrown dailies and Argent Tournament dailies. There wasn't all that much else.
That's why I disagree WOTLK was best with this kind of wording.
To each his own I guess.Pandaria should be last on the list.
IMO Pandaria was one of the better expansions as well. Especially considering endgame world content. You had a lot of diverse dailies, you had the farm which was entertaining and profitable without being extremely mandatory (like garrisons are); you had Timeless Isle at the end and Thunder Isle before that as major quest hubs.
Raids in Pandaria were also quite decent (ToT and SoO were both really entertaining raids; the earlier ones a bit less so IMO).
The biggest letdown in Pandaria for me personally was that the whole lore thing didn't appeal to me. I have nothing with Pandaren other than that the brewmaster was a nice easter egg in WC3.