Wow. Big question.
Well, I was a kid in the 80's. There was a corner store at the end of my street with some pretty cool owners. They also had games at the nearby supermarket. Over the years, they had:
Super Mario Brothers
Star Wars X-wing
Indiana Jones & Temple of Doom
Black Tiger (very cool sidescroller, better than Rastan)
Gauntlet
Pac-Man
Frogger
Rolling Thunder
Outrun and others
There would be an all-out race when school got out to the Super Mario game. That game was insidiously popular, even before the NES. My favorites from those days were
Indiana Jones, Star Wars, Gauntlet and Black Tiger.
On the PC front:
Starflight (untold hours completely absorbed in this one)
Earl Weaver Baseball (had a lot of fun playing this with my brother. I remember the day my Dad shelled out around $200 (of 1988-89 dollars - closer to $350 today!) to upgrade the Tandy from 256 to 384k, which allowed EWB to run in 16 colors rather than 4. He was actually pretty conservative, and this was a very rare instance. I was shocked. But there was some method to the madness. I then began programming graphics in 16 colors as I struggled to approximate the graphics of the day.)
Space Quest I - "It was not known that you are a master of the rock".
Pool of Radiance - It took, literally, several hours to install on the 8088 and required the shuffling of at least a half-dozen floppies. Never finished the game, but it holds a special place in the D&D geek pantheon.
More recently, I found GTA3 to be truly fun, and not at all the negative story-line the media seized on. Your character is basically an anti-hero, doesn't do drugs, etc. Never finished it, but when I think of the epic chases, the FBI and the helicopters coming after you, driving your stolen SUV off a cliff into a river... good times.
Oblivion was pretty fun in the main plot.
I found a copy of Bridge Commander for $5. Along with Hidden Evil they were some guilty pleasures.
LOTRO is my first, and only, MMORPG. I haven't gotten into it that much, but when you find another player or two that role-play a bit, its great fun. It's also pleasingly easy on the system requirements, looking great on an X700 and single-core processor @2.2ghz