Probably the best game of all time. God tier single player and multiplayer.
Easier choice when you factor in influence, all the mods/games it spawned, how it basically paved the way for Valve/Steam, etc.
There is no game I have spent more time in than this. The second closest game probably isn't even fucking close. I played this game regularly for YEARS.
HL2 was excellent, but it still wasn't as good as the first game. The first game was just too perfect.