Zul'gurub. I was in a Dutch social guild, they were level 70, and I was level 57 (or something) but they 'allowed to come along'. Because they were practically incapable of performing at their level, they sometimes did old-world raids for fun. Despite them being grossly overgeared they still had to explain tactics and even managed to wipe on some bosses.
Once I reached level 70, I did some research on the best-in-slot pre-raid gear I could get (from dungeons, quests etc.), and was the best-geared person in the guild shortly after. I then raided Karazhan with them (horribly slow raidleading, poor performance; only downed the first few bosses). I took it upon myself to assist in explaining the fights and raidleading, since I pretty much knew what to do better than they did. At one point we had a raid without some of the "bad apples" in it. It went really well, and we downed some bosses we never downed before. We went on raiding for a while beyond scheduled raid time, and actually had fun for a change. As a result of this, I was demoted from leading raids, because there was "less bosses left for the second raid that week" due to us going on beyond scheduled raid time. (Bad apples in charge held a grudge for it going so well without them.) I couldn't accept that fun and progress were secondary to raid schedule if everyone in the raid had wanted to continue (as was the case), so I applied for a more 'serious' raiding guild and left on more or less friendly terms. I just outgrew them, they were holding me back.
Ended up in a raiding guild with which I raided Gruul's lair/Magtheridon/SSC/TK, BT and eventually Sunwell (where we stranded and I finally stopped raiding). The raiding period I had with this guild was immensely fun, although I gave up raiding afterwards (don't want to have multiple evenings scheduled while working a day job). It was the most fun I had in the game, I think. Mostly because the raid leaders were great fun (nice guys, Brittish sarcasm and lots of laughs).