AgtFox

Doing a top 10 is very hard, so I will give a list of 10 for PC adventure games (my favorite), all other PC games and for consoles pre-Playstation and post-Playstation.

PC Adventure Games (ok...so it's over 10 titles)
Day of the Tentacle
The Dig
Gabriel Knight: Sins of the Father
Gabriel Knight II: The Beast Within (the best FMV-based game)
Gabriel Knight III: Blood of the Sacred, Blood of the Damned
Grim Fandango
Indiana Jones and the Fate of Atlantis
Monkey Island games (1, 2, Curse and Escape)
Sam and Max Hit the Road
Space Quest IV (the best of the series)

Non-Adventure PC Games
Civilization series
Deus Ex
Half-Life
Heroes of Might and Magic 3
Masters of Orion series
M.U.L.E.
Planescape: Torment
System Shock 2
Ultima IV and VI
Zork series

Pre-Playstation Video Games
1t. Legend of Zelda: Link to the Past (SNES)
1t. Super Metroid (SNES)
Bionic Commando (NES)
Castlevania 3 (NES)
Chrono Trigger (SNES)
Final Fantasy VI (or III in the US - SNES)
Final Fantasy IV (or II in the US - SNES)
Legend of Zelda (NES)
Super Mario World (SNES)
Yars Revenge (Atari 2600)

Post-Playstation Games (more than 10)
1. Castlevania: Symphony of the Night
Chrono Cross (PS1)
Final Fantasy IX (PS1)
Final Fantasy X (PS2)
Grand Theft Auto 3 (PS2)
Halo (X-Box)
Metal Gear Solid (PS2)
Metal Gear Solid 2 (PS2)
Legend of Zelda: Ocarina of Time (N64)
Legend of Zelda: Majora's Mask (N64)
Soul Calibur (DC)

TeeSeeJay

Ultima V (PC)
Pool Of Radiance (PC) <-- Not the new one
Under A Killing Moon (PC)
Wing Commander (PC)
Descent (PC)
Command & Conquer (PC)
Tetris (Multi-Platform)
SimCity (PC)
The Sims (PC)
Ultima VII (PC)

My tastes tend to run towards PC RPG games, but I tried to list a number of genres.

It's good to see that many in this thread mention Tetris. When it comes to mindless time-wasting, it doesn't get any better than that, and it can be pretty much played anytime, anywhere, on anything.

AndyCapps

1. Mortal Kombat (Arcade)
2. House of the Dead 2(Arcade and Dreamcast)
3. Off Road (Arcade)
4. Metal Slug (Arcade)
5. Wrestlefest (Arcade)
6. Quake (PC)
7. Waverace (N64)
8. Spy Hunter (Arcade)
9. Pac Man (Atari)
10. Castlevania (NES)

This is my "Top 10 that I can think of offhand" list. I left out Tecmo Bowl and RC Pro Am... and I skipped my Genesis years.

Eternia

1 - Super Mario 3 (NES)
2 - The Legend of Zelda (NES)
3 - Wizards and Warriors (NES)
4 - Dark Castle (Mac)
5 - Super Mario Kart (SNES)
6 - Super Mario World (SNES)
7 - Sim Tower (PC/Mac)
8 - Gauntlet (NES)
9 - Gauntlet Legends (N64)
10 - Super Ghouls and Ghosts (SNES)