Well I have a 1TB drive just for games so they don't tend to get uninstalled, but for the sake of the thread...
1: Super Street Fighter IV: Arcade Edition 2012 - best fighting game of all time so it will not get uninstalled, at least until Super Street Fighter IV: Turbo Hyper Fighting Alpha *
2: Left 4 Dead (1 and 2) always a good multiplayer game and needs to be available at a moments notice.
3: The Ultimate Doom: Thy Flesh Consumed - The best version of Doom, never know when I will have a run through that one.
4: Magicka - For the same reasons as Left 4 Dead.
*Name may not be 100% accurate, Street Fighter X Tekken has not made me want to uninstall SSFIV:AA