Half Life, for being the first first person game I've ever played on PC. Right after it comes Project Reality, a mod that was developed for the Battlefield 2. It was the first game (mod) to introduce me to an awesome online community. And last, but not least, Team Fortress 2, for giving me a best friend with whom I still talk to after almost 10 years! I don't play these games anymore, but they will always be my favorite games due to their emotional significante and of course, no other game is or will be able to occupy these positions now.