Dead Space, because I love that game.
Borderlands, 100% if you don't count the DLC, game got a little too repetative and didn't see any desire to get DLV.
Transformers WFC, 100% baby, because it is by far the best Transformers game in existance, and had nothing to do with Bay-formers.
Defense Grid, 100% including all the DLC, without using the internet to gold metal the maps (except for one map).
I am one chevy away from 100% on Geometry Wars: Evolved. And if you have played that game, you know what achievement I am missing... "Smile". VERY hard to get.
Also very close on Batman AA, about halfway through beating it on hard and one more challenge map to 100%, but where will I find the time...
EDIT: oh and RE5 is you don't count DLC.