My proudest moment was when I killed Lich King on normal 10 man.
My guild at the time was alright, 2 nights a week and we were clearing ICC minus LK with a couple heroics. The problem was, the officers had somehow gotten the worst strategy ever for LK phase 1. Essentially they had everyone stack, and they would dispell Necrotic Plague until it left. They would AoE the adds down as much as they could, and finish them off in the Transition Phase, and then have a couple Raging Spirits up in Phase 2 with the Val'kyr. They somehow got this ridiculous strategy to work in 10 man, but it just wasn't working in 25 man and we were wiping on Phase 1 of the fight like crazy, despite the protests of the strategy. Like most guilds, it was full of drama and politics. We had 9 officers, and the 9 officers plus their 1 friend would run that 10 man, and then just use the rest of us for the 25s.
One night, I got in a 10 man PuG that was going to be doing a few heroics. It was a really good group, and really nice guys all working together. We go to LK doing like 3 or 4 heroics, and it was really late. We decided to go for it, and after like 4-5 attempts we got it, and the kill attempt was so insanely close I still can't believe we survived. I was just so happy that we had accomplished something together, and I had done it without the ridiculousness of my guild at the time. I couldn't sleep that night because I was so happy I had finally accomplished something in WoW. Needless to say, the guild I was in didn't last much longer.