Half-Life 2 for single player. The ending was too short and too easy, but that is literally my ONLY complaint. It has such great characters that a robotic dog with no dialogue is more believable and fleshed out than most protagonist humans in "realistic" games. Also, anyone but Valve would have been so pleased with themselves over the Gravity Gun that they never would have gotten to the Pheropods, one of my favorite weapons of all time.
I would say Counter-Strike for multi-player but that's just because I gave up on FPS multi-player when I got sick of constant PC upgrades and switched to console gaming. Never got into Live, but that's a whole different thread.