Lotta cool picks so far!
Doom and Duke Nukem 3D. Doom just holds up tremendously, and is crazy how innovative it was. And Duke 3D is my fav for that era and style of level design, just perfect.
GTA3 and Vice City. GTA3 knocked my socks off, invented a new subgenre pretty much. Still fun today. Vice City perfected that in my book, so smooth all around. And the series overall, I'd call GTAV a masterpiece as well for sure. The sandbox with a fun city to explore, and an engaging and funny main adventure.
Space Quest IV and Quest for Glory (VGA). Definitive point and click adventures for me, so much character and inspiration. Space Quest IV feels you're controlling a character in a movie, especially at the time. And QfG, speaking of Witcher 3 (which rules), a lot of what Witcher 3 does is done in a smaller scale in Quest for Glory. Really captured my imagination, seemed everybody who played it enjoyed making a paper map of the forest screens. I sure did.
Sonic the Hedgehog and Sonic 2. Agreed Mario is a series with many masterpieces, and Sonic has some in his own right. These classics put a nice spin (heyooo) on the genre.
Last but not least, I'll say Metal Gear Solid 3, could be any of them, but this one is a fun fav.