Doom 3 may be the most taxing for a game, but the game play sucks. It gets repetitive after half of the game. You almost know where zombies are going to walk out and come after you. On the other hand, Half-Life 2 lets you tackle things differently and the way you want to. Becides, Half-Life 2 probably has the best physics for a game ever. Even Doom 3 doesn't have that good of physics. Even though Doom 3 requires more from the system it doesn't mean that the game is better.