I know most of you will think I am out of my mind, but Shadowrun the FPS cross platform game is easily hands down my all time favorite game. It's the one game i can enjoy playing at any time. Sure I wish it had some updates, but even without them I usually have a blast playing it.
That said, there are a couple other games I think are really up there on my list of best games, BUT if I had to choose, the original Half-Life would be among the best of them, if not for only the game itself, but the dozens of awesome mods that sprung from it (including the original Counter-Strike).