Single Player: Half life 2, I loved the bright outside levels, the AI, the atmosphere, the physics, the pace, and I thought it had a great story, not quite as good as original Half-life but better than almost any other gamne I have played. the outside levels and atmosphere have to be the thing that sets HL2 above HL for me but HL is way up near the top of my best games list
Multiplayer: BF1942 Desert Combat, I have never had as much fun at a lan as back when we used to play DC, I loved the helicopters and the cruise missles and how the artillery worked and how I had a friend who whenever he was on the other we name himself the "anti-spectre missle" and just get into planes and crash into me to prevent me from doing massive damge with the helicopter, I also loved how big and open the levels were, there was lots and lots of space to mess around before you went out of the map boundaries