That simple. Post the Top 10 of your favourite games here. Any explanations for a choice of a game are appreciated. If you can't get 10 games, or if there are more then 10 games, no problem with that, just post what you can. This way we may see what people here like to play and maybe somebody will discover an interesting game she/he haven't played yet.
Here's my Top 10, in no particular order (can't really tell what game is the best one from those listed):
1. Planescape: Torment
RPG
Simply the best storyline, universe (coming from an D&D setting - Planescape) and characters in a computer game to date IMHO. Kind of like Final Fantasy, but much deeper and more philosophicial and the game itself is far more an actual RPG game then a sequence of animations / predefined dialogs separated by random battles. If a game can change somebody's views on the things surrounding you, it is allready a lot and something quite unique. I know that it changed me...
2. Fallout / Fallout 2
RPG
Best CRPG ever as far as gameplay matters. A serious post-nuclear setting mixed with a bit of humor and sarcasm in the story, a very good role-playing system and interesting battles, lots of freedom and ways to complete quests, and tons of guns

Each part of the game took at least a month of my life...
3. Thief: The Dark Project / Thief 2: The Metal Age
Action
Unique gameplay, dark fantasy setting, and most important an amazing surrounding and the effect it has on the psyche - there were really few games that made me feel fear. The game certainly gets my award for best sound too.
4. System Shock / System Shock 2
Action/RPG
What was said about Thief can be said about this game as well. But this time it takes place in a cyberpunk setting. This results in the changes in gameplay, but incredible surrounding and the feel of fear is still there. No wonder, the games were made by almost the same team... Damn monkey... where's my shotgun?
5. Deus Ex
Action/RPG
Kind of similar to System Shock in gameplay, but very different at the same time. The feel of fear is not as strong, but instead of it there's a great storyline filled with conspiracy theories.
6. X-Com: Apocalypse
Strategy/Tactics
The best game in X-Com series, with X-Com: Terror from the Deep and X-Com: UFO Defense needing to be mentioned (X-Com: Interceptor and X-Com: Enforcer do not deserve it - they were just horrible...). Best tactical game I have ever seen (and I've seen a lot). Turn-based or real-time depending on your preferense (screw the real-time, turn-based all the way

). Brilliant and very realistic (even though it is a sci-fi game) combats, huge maps where you can level down the whole buildings with your explosives. This game earns the prize for taking the biggest chunk of my life...
7. Diablo II
Hack and Slash RPG
I think everybody knows this game... not a lot need to be said here...
8. Heroes of Might & Magic 4
Turn-Based Strategy
Some people will wonder why only the 4th game is mentioned. It's simple - it is the first part of the game (and I played all four) that I liked enough for it to get to Top 10.
9. Neverwinter Nights
RPG
This is not just a game, it is a constructor for games. The whole project was initially started with the idea of it to be a tool for people to create their own modules. Therefore how good the game is depends a lot on the module you play, but the tools themselves are just great, and developers give a huge support to it.
10. Baldur's Gate / Baldur's Gate 2 / Icewind Dale / Icewind Dale 2
RPG
All this games are mentioned together, because they are very similar and use the same engine. They all are just good CRPGs with Icewind Dale series being a bit more of hack-and-slash-like games.
Feel the need to sneak Elder Scrolls 2: Daggerfall and Elder Scrolls 3: Morrowind into Top 10, but can't really find a place... so I just mention them...