I don't mean this as a "play version X instead of version Y", I mean just in general people should play Quake 2 PC version. It's an excellent game, it's rock solid fun. In fact it's literally more consistently high-quality than Half-Life 1. HL1 was awesome however it has some janky inconsistent levels because of the long development process. Quake 2 is beautiful crunchy fun from the first second to the last second.
I think Quake 2 was released for free, too, so it's easy to get. Though I forget if the game assets/art-files was freely released or only the source code. In any case it's easy to find, and easy to run.
Wonderful PC game. And Yeah I'm talking about single-player Quake 2. Multiplayer was the golden days, but you won't find an active community this century, so just play the single-player.