"Herald of the Titans" - 11/04/09

(Defeat Algalon the Observer in 10-player mode at level 80 without anyone in the raid wearing any equipment with an item level higher than is available in 10-player Ulduar.)

That's my favorite achievement. We were a 10-man strict guild back then and worked so hard to get it. Coincidentally, we are also the only 10 people on our server to hold that title, so that makes it extra special. Every boss kill since then has felt shallow.