Do I have to limit it to 10?
Okay, I'll do my best.
*downloads VICE and DOSBox*
The Last Ninja (C64) (Game frustrating with the leap on that tiny rock, but other than that, it's one of my top 3 choices for the C-64)
TIE Fighter (DOS) (Loved this game tons more than x-wing, mostly because of the better damage statuses)
Master of Magic (DOS) (Magic meets Civ, I love this game)
UFO: Enemy Unknown (X-COM1) (DOS) (Micromanagement isn't for everyone, but this had the right level of it for me)
X-COM:Terror from the Deep (DOS) (Not that it's a great sequel, there's aspects of this one which I haven't mastered to this day)
Neverwinter Nights 2 (It has a good construction set.)
Minecrack (I can't get bored of this game, but I wished that they'd allow a full revert because the latest update trashed the framerates on my PC)
Diablo II (I still have plenty I want to do in this game.)
Master of Orion II (Star Trek meets Civ, I prefer the music and the fact that star systems can have more than one planet)
Need for Speed Underground 2 (I put this game aside, but the game is great for everything but the drift events)
Although titles like Ultima IV, V, and Wasteland come to mind, I'm pretty sure they'd sit on my HD unexecuted for a long time after I've played them through once or twice. More.