In PvE it was probably actually a 10 man Sindragosa kill of mine. At the time, I was raiding in the 3rd ranked guild on my server, and we raided 25M HM ICC. It was right around the time my guild was progressing on Sindragosa, but for whatever reason, I can't remember, I had to take off raiding that week, or at least our raid nights. I think it was a Monday, and I was trying to get my HM ICC clear out of the way before reset. I started this 10 man PuG that was full of weirdly geared, sub-par raiders. It was basically that classic movie-type "group of misfits" thing going on. Anyway, we make it to Sindragosa, mind you a boss that I'd been wiping on in 25M HM for weeks but still hadn't killed, because I missed the guild kill. Anyway we go at this bitch for like 4 and a half hours and somehow the stars align and we kill her. It was still before the debuff and all that so to get a PuG doing that shit on my server was unheard of. Pretty epic.
PvP i couldn't even begin to pick just one. I've had so many crazy 1v2's and shit in arena and such. Pretty sweet though.