I don't really hate any raid, but these I enjoyed the least
1- DS
2- ToC
3 would be probably be Hyjal and MC, but I didn't do those when they were current, so I won't judge.
I don't really get the hate for Nax in Wrath. I personally started about halfway in TBC and hit a huge progression roadblock because that path was already gone at that period in the expansion. Then comes WotLK with a starter instance which was great if you were fairly new to raiding. I was in a casual guild back then and was just asked to tag along regardless of me having my doubts about going their in my gear. Believe me, we couldn't clear that instance on 25. Maybe it was easy for all those raid veterans, but we never killed Thaddius if we even could get past Gluth (hell we had some blocks on Patch as well). Later we cleared it on 10 with the better people which we later took to Ulduar in all it's awesomeness (and lets face it, Ulduar was quite a decent challenge). The thing of Naxx I liked was that it allowed you to get better at the game because you had ACCESS to it. Not able to do raids because you couldn't get your gear up to scratch anymore was not fun.
As for the 'rehashing'... for most people it was new content.