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)

:-)

1. Wolfenstein 3D
2. The Ultimate DOOM
3. Sonic CD
4. Heretic
5. Hexen
6. Quake
7. Diablo
8. Blood
9. Carmageddon
10. POSTAL
11. Unreal Gold
12. Gran Turismo
13. Aliens Versus Predator
14. Dino Crisis
15. Grand Theft Auto III
16. Silent Hill 3
17. Devil May Cry 3
18. F.E.A.R.
19. Risk of Rain
20. Shadow Warrior 2

I need to play more video games.

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