This is a "work in progress" as I have played literally over 150 full games to finish over the years. I haven't played The Witcher 2, Civ 5, or Total war Shogun yet all of which I own, and hopefully may be in the top ten.
For now I'll just list a few of my most memorable experiences and update later.
Total War - Rome
Mass effect 2
Elder scrolls 3 - Morrowind
Elder scrolls 4 - Oblivion
Duke Nukem 3D (original)
I'll add a few more after a good think, (I feel I've missed some out) and set about the very difficult task of listing them from 1-10.
Good thread Frankie! I hope more share their gaming memories with us!