1. Command & Conquer - the original and best. I liked Dune II but C&C was just brilliant. I can still remember every mission for both sides in full detail and could probably tell you every crate location too (I played this game a lot). The first time you faced an obelisk of light you knew true fear, even now that charging up sound sends shivers down my spine. The FMVs and digital music were just the icing on the cake. The first game I ever played multi-player on a PC with a null modem and every single battle was like a game of chess. My favorite game ever.
2. Half-Life - scary and awesome, like a film even though there were no characters. The ending sucked but who cares. If you played it, then you'll remember the first time you faced the grunts, the first time a headcrab bit your face in a vent shaft, the first time you got chopped into bits by falling into a fan... I must've spent/wasted hours playing Team Fortress online and I liked Counter-Strike too until the cheaters took over.
3. Deus Ex - one of the finest stories ever in a game, a real adult thriller. The roll-playing elements made it excellent and the way that most missions had at least three possible ways of winning was really fresh. Also 3 different possible endings depending on your political leanings (not including the secret dance level). I really liked the way that you could fail certain objectives too, without the game just stopping for no reason, although the grief you got on returning to base made you desperate not to fail. The characters really come to life in this one and I don't think I've ever played it through without finding something new.
4. The Elder Scrolls: Morrowind - if anyone has completed every mission in this game I want to shake their hand. This game was HUGE and yo could complete it by only doing about 5% of the missions. Fantastic story, excellent character development and I loved the way that you would feel like you were there and it was real even though it had a big fantasy setting. Completely non-linear too, the best bits (if you had a good enough graphics card) were when you left the track and found yourself looking over a gorgeous valley in spring (yes it had seasons) or a seething caldera.
5. Shogun: Total War - I wouldn't be posting here if it wasn't for this game. Changed the world of strategy games forever and spawned Medieval and Rome. A faltless epic.
6. Caesar II - would it be a little cheeky to suggest this game pre-empted Shogun's concept by quite a few years? I loved the fantastic mix between Sim City style city building and real-time battles.
7. Mafia - guest director Martin Scorsese? The best story in any game ever, this is a film in so many ways - the characters developed brilliantly throughout the story and even the city itself advanced and grew as the game went on. Perhaps the gameplay wasn't the best ever but the story made me just want to keep playing on. As for the ending... perfection.
8. Civilisation II - need I say more?
9. Age of Empires - once the Rise of Rome expansion was realeased this game just exploded into the greatest build 'em up and smash 'em down strategy game ever. The historical aspect of the game made R:TW look like an Encyclopedia but no one cared. Still hugely popular on the zone today.
10. Championship Manager 98 - this game stole several months of my life that I want back. I don't know why its good and I can't explain why I kept playing it, but it must be something special to keep my attention as long as this game did.