What is a 10/10 game? Everyone should describe that before putting their answers. Because I doubt that a 10/10 game means a game with no flaws. So there needs to be description, parameters.
For me, a 10/10 game is a game that makes me say "wow" the moment I start playing it - and continues to make me say "wow" after I finish it. A 10/10 game is a step above a merely excellent 9/10 game; a paragon of its genre, a triumphant moment in gaming that will either stand the test of time, or will be remembered fondly. In other words, a truly special game.
With that in mind, here's why I think these 5 games deserve 10/10.
Unreal Tournament 2004: endlessly fun, easy to learn yet well-balanced and difficult to master gameplay. Graphics that is at once technically impressive and aesthetically gorgeous, even to this day. The sheer volume of content sealed it. This game was built to stand the test of time, and it does.
Sonic the Hedgehog 3 & Knuckles: 2D Sonic platforming game formula perfected, then combined with an staggering volume of content.
Portal and Portal 2: simply put, I've never had so much joy playing a video game as I did playing these two games.
System Shock 2: a case study in horror gaming that doesn't rely on annoying "can't see s***" lighting and tiresome jump scares, which people are still trying to imitate and failing miserably to this day. Atmospheric, immersive, terrifying, memorable.