Let's see... I'm probably most proud of my "Strange Relic" Trophy in Uncharted. The relic itself is really cool because it's in the form of a Precursor Orb, which is a collectible item in the Jak and Daxter games. HOORAY FOR CAMEOS!!!

I got that same Trophy, Doc. I didn't think I'd get it so quickly, though.

I know 100 headshots may not seem like much of an achievement, but LittleBigPlanet and Uncharted are the only two games I have that have trophy support.
Speaking of LBP, I'm also pretty proud of achieving the "Incredible Height" and "Incredible Speed" trophies which I both managed to gain in one shot when I built my own rocket, blasted off in it, and then got blown right off of it when it crashed into a wall, which caused me to go flying through the air and hit the ground at at least 200 miles an hour! Booyah! Two trophies at once!

Going over the finish line on foot on Banjo Kazooie N&B also while driving backwards.
I even waited till the last second before someone caught up to rub it in more.
Most people who have that game have not gotten those two achievements.

Owning all Zelda games in existence, excluding that stupid Tingle game on the DS and those excuse-for-LoZ-games by Sony.

Oh, and playing Fable 1 for 15 hours straight.

Maybe not official achievements in Xbox terms, but I'm happy to have achieved it.

Errmmm. Mile high club on COD4. Man vs Tank (kill a tank singlehanded) Left for dead. and finish all songs on expert drums rock band 1 (Run to the hill was my main moment of joy).

OH! Thanks for reminding me, rookyboy! I remember getting an achievement on Rock Band for clearing all songs on Expert (Vocals) and getting the highest score in a band on Guitar Hero: World Tour.

BTW, the Zelda CD-i games were developed by Philips. Not Sony.

Oh whatever, they were still crap :P

Probably finishing 7 Kombo challenges in MK vs. DCU and crossing the finish line on foot like PrinceKiseki said.