World of Warcraft - if by a small group, put on a top #10 Paladin list of a very heavily populated server (Blackrock) in '05 or early '06. Numerous outnumbered odds wins in PvP, etc.
Jedi Academy - against a player better than me 1v1 and one almost as good as me, lost 5-3 in a 2v1. Also one of the best Aussie players back in the day; I've since been replaced.
Team Fortress 2 (PC) - 13491 healing as medic without dying on Dustbowl defence cap 6. It was a public server, but hey! It's on my record, now.
CoD4 - finished Veteran difficulty in less than two days with no prior experience with the game. Not as good as the JKA / WoW ones, but it's still pretty recent.
For the most part, I'm not really a collector or anything when it comes to singleplayer games, with Bioware products being the exception. I try to complete everything I can in them - get the best gear and complete all the sidequests before moving onto the last bits. Final Fantasy games, however, I just can't finish - I've come close to doing so, but I always get distracted by the huge amount of side content there is. Side content I'm not good enough to complete. Once my 360 is back from repairs, I'll do an Insanity difficulty run of Mass Effect with a new character; should be able to add that to my list of achievements.