Lets see... It's not realy a list where 1 is better than anything else, but more of the 10 games I continuosly play, no matter how many years have passed.
1 ) Civilization -> from the first one up to version IV. I lost count of how many hours i've spent moving settlers, planes, ships and armys around.
Proud victim of the "Just One More Turn Syndrome".
2 ) SimCity -> again, every game in the series.
3 ) X-Wing, Tie Fighter, XvT and X-Wing Alliance. Well
These will forever have a special place in my heart. Also because XvT was the first game I ever played online.
4 ) Flight Simulator -> I always wanted to become a real pilot, but for "medical" reasons, I can't. So PC simulations are better than nothing. Starting from FS 4.0 up to lastest installment.
5 ) Deus Ex -> absolutely the BEST scifi fps/rpg ever. And for me beeing a fanatic of post-apocaliptic-bladerunnerish settings, it's easy to understand why
6 ) Homeworld 1 -> I had tears the first time a burning Kharak apperared on the screen.
7 ) Star Control 2 -> Brilliant game, from the fog of time, when computer games where something "special" and personal.
8 ) FreeSpace (and SCP) -> a bit low in the list but still present on my hard-disk.
9 ) Super Mario Bros. 3. There's a long rivalry between me and my sister: I still haven't beaten her after more than 17 years. So it deserves a place here.
10) Hearts of Iron 2. I like all the Paradox games in general, but HoI 2 is still the one I fire up the most.