Unreal Tournament... So I'm already satisfying my need for a little nostalgia :D
When I was a kid I think my favourite game was always Chaos on the ZX Spectrum (nowadays the whole system can run in a JS-based emulator so you can play it online). I'm old enough to remember playing Binatone pong on a TV; so people can quit with the "I feel old" when someone mentions games from the 90s :)
Other retro PC recommendations:
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