Team Fortress 2. Portal was good but too short, Half-life 2 had a good story but mediocre gameplay, Counter Strike sounds like an average shooter that is multiplayer only (well balanced does not count as an innovation, but I could be wrong about this because I've never played CS), Left 4 Dead got repetitive quickly in human mode, but undead mode was great (they should have expanded upon it more). Team Fortress 2 had balance, classes, humor, atmosphere, and great gameplay, and I consider kit to be one of the best FPS games.