I remember I played MM VI and VII one after the other in 2001, a couple of years after their official release. At first I did not think much about them, but after a few weeks... oh boy... what an incredible experience. I played again VI last year and although the graphic sucks I spent another 100 hrs with it.
Come to think of it, the games I had little expectations ended up to be some of the best for me. Recently, D:OS. After Divine Divinity and Beyond Divinity, I had very little expectations for D:OS. Oh boy, was I wrong....
Some of the others, in no particular order: Eador (if you like strategy, don't be a fool, this one is probably the best fantasy 4x ever made), Deus Ex (the original), Fallout (yeaph, snatched from a garage sale for a few cents...), UFO Enemy Unknown (the first time I played I disliked it so much I did not touch it again for months...), Heroes of M&M (I started with 2), Planescape (this is sort of a miracle for me, since I hate RTwP with a passion, and yet this one is at the top of my list), Valkyria Chronicles (I don't even like JRPGs)....