Games I spent waaay too much time on - 80s onwards.
Elite - Spectrum/Electron/C64
Need I say more?
Commando - Spectrum
This was extremely playable. I loved this game.
Lightcycles - Spectrum
Ridiculously simple game and excellent fun against a mate. Sharing a ZX Spectrum keyboard though...
Ghost Busters - C64. 'He Slimed ME!' Discovered the easiest way to get through the first level was to get the Beetle and a couple of traps. You easily made the money to get through. (edit - actually this was technically the first game I completed, thinking about it)
Forbidden Forest - C64
I would go round to my mates, then subsequently ignore him whilst I played this over and over and over...sorry Dan. Excellent mood music.
Gunship 2000 - C64 and Spectrum 128, First 'simulator' I truly got hooked on. Remember the soldiers^h^h^h^h vertical lines?
Marble Madness - Spectrum. Again, very playable 3d(ish) platformer.
Spin Dizzy - Spectrum..same as above.
Nemesis - MSX. Hours turned into days turned into weeks lost on this. First game ever completed.
F1 Spirit - MSX. Probably the second game I ever completed.
Nemesis 2 - MSX. Not quite as many hours as first game, but still very fun.
Wipeout - Playstation. That soundtrack...those graphics and game play, I was Hooked. HOOKED.
There were obviously more, but these were the ones that stick out in my head. Battlefield 1942 in 2003 was the start of something darker and lead to the current game of choice, Ghosts. It is a marvel I ever got married, come to think of it...