I honestly can't choose, so I'll give my top few. In no particular order...
Star Wars: Knights of the Old Republic
Star Wars: X-Wing Alliance
Star Wars: Dark Forces II- Jedi Knight
Command and Conquer Red Alert 2
Descent: Freespace
Crysis
Metroid Prime 3: Corruption
And probably a few more, that I can't remember off hand.
i think the first game i ever played was lemmings or humpty dumpty on an ancient pc, but the top 10 were: (in the order that i played)
1. the original red alert with the four coloured discs (i still have it) PC
2. starcraft PC (me and my older bro were very competitive)
3. Starfox N64 [like freelancer only with spinning ships;)]
4. mariokart N64 (only ever played N64 in a friends house, still probably logged a few days worth of gameplay)
5. Red alert 2 + Yuris revenge expansion
6. Unreal Tournament 2004 (used to play on a LAN in my school with about 10 friends everyday against each other and the sixth formers - good times)
7. Super smash bros. brawl (gamecube) no - one would play with me in the end lol
8. Oblivion - thankfully i lost my obsession when the shivering isles expansion gave me a bitter taste in my mouth (metaphorically speaking of course)
9. Rappelz (free WoW style MMO got obsessed)
10. freelancer discovery - the latest and perhaps most obsessive bar starcraft (i missed school for that many times)
@retnuh I second all the 64 titles, starcraft, the red alerts, but mostly brawl, the best smash brothers game ever. I probably wasted months of my life away on that game in college. Although was it really a waste if there was nothing else to do and hardly any women... I Think Not!