#1: Deus Ex - will forever and ever be my favorite game of all time. I was already really enjoying it for the first few hours but by the time I got to the Hell's Kitchen level I had practically fallen in love with it. Some of the smartest, most thought-provoking and most compelling writing in all of gaming.
#2: Team Fortress 2 - this game just makes me feel happy. Whenever I'm feeling down I can just hop into a community server or a Casual match and just have fun. I hope it never dies.
#3: Saints Row 2 - very compelling open world, very compelling story, incredibly stupid gameplay that invariably makes me chuckle every time I decide to play it.
#4: Vampire: Bloodlines - I just beat this game for the first time a few weeks ago and it has already found a place within my heart. Gameplay is fairly jank but it more than makes up for it with its aesthetic, storytelling and character development. Everyone in that game feels human even though they're dead.
#5: System Shock 2 - another really janky game, but one that is very effective in terms of horror and storytelling. Sound design is top notch, and the environment does a lot to make itself feel way bigger than it actually is.