By which I mean any game released in the '90s, not just games you played back then. If you were alive.
My favourite is Half-Life. It was a revelation in terms of grounding in a game, with no cutscenes, and an amazing fusion of setting, gameplay and the same grounding keeping it all extremely cohesive. It's probably my favourite single-player game of all time.
Other favourites of mine include:
Myst
Riven
Crash Bandicoot 3
Quake
Spyro 2
Ape Escape
Banjo-Kazooie (tried it on XBLA, it's easily the equal of my beloved PS1 platformers)
P.S. I've never played Ocarina of Time, Mario 64 or Grim Fandango (that said, GF is coming to PS4 at some point. :D ).
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PC: Counter-Strike PS1: toss-up between Resident Evil and Tomb Raider (would have been Legend of Dragoon but it didn't release until 2000) NES: Super Mario Bros. 3 TurboGrafx-16: Bonk's Adventure Sega Genesis: Mortal Kombat 2: Annihilation Super NES: Super Mario World N64: Golden Eye 007