1. Witcher/Witcher 2
2. Planescape: Torment
3. Starsiege: Tribes
4. AoE 2
5. DA:O
6. DE/DE:HR
7. Knights of Honor
8. BG1
9. SWAT 4
10. NFS 3
Hmm, list hasn't changed much in these last... 8 (... holy shit) years.
I guess I'll update it and expand a bit:
1. Witcher/Witcher 2/Witcher 3 - Now including The Witcher 3! Each game in the series tried to do different things and is quite different from the other ones, but they all maintain a very strong storytelling core that keeps them at the top of my list. Impossible to pick a favourite between these three.
2. Planescape: Torment - A milestone in cRPGs with a narrative focus. The Nameless One is an excellent video game protagonist, turning the cliched "protagonist with amnesia" premise on its head. The world of Planescape is also one worth experiencing. A shame the recent spritual successor - Tides of Numenera - missed the mark. Disco Elysium is an honorable mention came closer than Tides of Numenera, and probably would have made the list had it come out 20 years ago.
3. Starsiege: Tribes - A very unique multiplayer game that combines jetpacks, speed and skill shots with slow-moving projectiles to produce an FPS experience like no other. Tribes: Ascend is an honorable mention that tried to revive the franchise, but ultimately failed thanks to a horrendous F2P model.
4. AoE 2 - Well, nowadays you have the Definitive Edition, with a lot of new civs and campaigns. The fact that this game is still one of the most-played RTS games today is a testament to how good it is/was.
5. DA:O - Took the ideas from Baldur's Gate and elevated them to the modern (at the time) era, while also innovating in terms of character creation. Hnorable mention to Divinity: Original Sin 2 which took a similar approach (a weaker story, writing and presentation is why it's only an honorable mention).
6. DE/DE:HR - The definitive immersive sim franchise (with some highs and lows). A shame Mankind Divided was such a let-down, the game had huge potential.
7. Knights of Honor - Total War with a world map that updates in real time rather than using turns, with somewhat deeper kingdom management (though not quite as deep as Crusader Kings). I've sunk so many hours into this game that I had to put it on the list. Excellent soundtrack too (by the composer of the music for Divinity: Original Sin 2 and the upcoming Baldur's Gate 3). WIth Knights of Honor 2 around the corner, I am very excited.
8. BG1 - The blueprint for most modern story-driven RPG games. Down on the list because of a weak narrative. Nonetheless, a memorable adventure in the Forgotten Realms and a landmark title. Let's hope Baldur's Gate 3 can surpass it when it comes out.
9. SWAT 4 - The best tactical shooter out there, even to this day. Honorable mention goes to Ready Or Not, which I understand is a modern spiritual successor. Have not played it yet, so I can't say if it belongs on the list or not, but definitely a game I am interested in playing once it's out of Early Access.
10. NFS 3 - To me, the best Need for Speed game. The only other meaningful innovation in the NFS franchise was Underground, which introduced an open world, but I didn't care that much for it and its nitro mechanic.