I'll try to play, although my mind is always changing. In no particular order:

Star Wars: The Arcade Game (arcade)
Contra (NES)
Doom (PC)
WWF No Mercy/Virtual Pro Wrestling 2 (N64)
Tempest 2000 (Jaguar)
Civilization 2 (PC)
Pirates! (PC)
Burgertime (arcade)
Pressure Cooker (2600)
Star Raiders (Atari 800)
Freedom Force (PC)
Ultima 3 (PC)
Sam and Max Hit the Road (PC)
Starflight (PC)
System Shock (PC)
Wasteland (PC)
Wizardry (PC)
Seven Cities of Gold (C64)
Metroid (NES)
Thief (PC)

Blood (PC / 1997)
Castlevania: Symphony of the Night (PS / 1997)
Command & Conquer - Red Alert 2 (PC / 2000)
Heretic: Shadow of the Serpent Riders (MS-DOS / 1994)
King of Fighters '98: The Slugfest (Arcade / 1998)
Legend of Zelda: A Link to the Past (SNES / 1992)
Mega Man X (SNES / 1993)
Metal Gear Solid 3: Snake Eater (PS2 / 2004)
Metal Slug 3 (Arcade / 2000)
Onimusha: Warlords (PS2 / 2001)
Quake (MS-DOS / 1996)
Resident Evil 3: Nemesis (PS / 1999)
Silent Hill 3 (PS2 / 2003)
Sonic Mania (PC/Switch / 2017)
Street Fighter - Alpha 3 (Arcade / 1998)
Super Mario World 2: Yoshi's Island (SNES / 1995)
Super Metroid (SNES / 1994)
Unreal Tournament (PC / 1999)
War for the Overworld (PC / 2015)
Witcher 3: Wild Hunt (PC / 2015)

(because I feel bad) 5 Honorable Mentions:

Company of Heroes (PC / 2006)
Crisis Core: Final Fantasy VII (PSP / 2008)
Torchlight II (PC / 2012)
Medieval II: Total War (PC / 2006)
XCOM 2 (PC / 2016)

:-)

A final, huge thanks to everyone who posted their top 20. I've discovered a few new games, and rediscovered a few more :)

Cheers - Chris