Ahem, this will be a little tough. I'm doing it in reverse order.
5. Age of Empires II: The Conquerors
AoE is an addiction. It is a phenomenon. Age of Empires II maintains a mechanical, evened-out game that only falls short of a sport. It requires luck, skill, brain and reflexes. It is the strange sort of game that no matter how many times you play, you just stay inside the game. AoE2 is the greatest strategy game I have ever played.
4. Knights of the Old Republic
I liked KotOR because it had the fun of a Neverwinter Nights with the adventure of LucasArt's classic adventure games, plus Star Wars. It did a great job of creating a lifelike world set in the Star Wars universe and yet so uniquely different. KotOR was fun, original and a dream come true for any SW fan.
I mention Half-Life because it is the First-Person Shooter. It had a gripping story, a never-ending sense of adventure, neckbreaking fun and the difficulty level was perfect. I haven't seen that sort of a difficulty level in any other game. Half-Life will definitely be one of the most memorable games I've ever played.
2. Deus Ex
Deus Ex is one of the most immersive games I have ever played. It is extensive, features a unique world and a brilliantly crafted story. It is the sort of game that makes you think. Deus Ex was fun, very replayable and a perfect fusion between FPS and RPG. I wish more games had been made using Deus Ex's original gameplay formula as a base.
1. Unreal Tournament 2004
Okay, no contest. UT is my love of life and I will be probably be playing it till I die, if the series lasts that long. UT is hectic, satisfying, 1337, fun - there are countless adjectives. UT is one of the most modified games around, it is fun every time you connect to a server, it's crazy. Buying a copy of UT is like buying a dozen first-person shooters - a massive bargain.