I'll recommend games to younger gamers but the graphics look like shit compared to today's games.

Unreal

Unreal Tournament (if you can find others to play with you)

Duke Nukem 3D

Sid Meyer's Civilization (the original one put out by Microprose. No DLC. Just one complete game.)

Doom

Quake

Might & Magic (yes there is no "Heroes" in the title)

Baldur's Gate (The very first one)

King's Quest

Simon the Sorcerer

MDK (Murder Death Kill)

Orcs vs Humans (when Blizzard were not a corrupt asshole of a company)