Oh, where to start.
Shadow of the Collosus is an absolutley gorgeous game. Beautiful graphics for the PS2, loved the boss fights, intuitive combat, Heart wrenching story. I'll be damned if this isn't the best fantasy game I've ever played.
Half life 1 is a good one as well. It was, essentially, what brought me into the next stage of nerdiness, and got me into PC gaming. The graphics aren't to bad, the combat is fun as mess, the AI is juuuust stupid enough, but still poses a challenge. I loved the story, and it had me glued to my shit laptop from the second I started it.
Dwarf Fortress is up there as well. It has a very steep learning curve, but once I got past that, It became a vast wonderland of Megaprojects, Ale, and Elf corpses. All rendered in beautiful ASCII
They Hunger is a magnificent mod for Half-Life 1, ranking up as my favorite single-player mod of all time. It has a great storyline, and really departs from the cliche that most zombie games set forth. It stayed true to it's timeline, And I never once thought I was shooting headcrab zombies in a re-skinned black mesa.
While I'm at it, Sweet Half-life is a good one as well. It takes the Half-Life engine to new heights, involving a branching story line and vast battlefields, as well as many factions to fight against. Of course, the Black Mesa crew gets a redub, with the Scientists, once pathetic wastes of CPU power, now with HEV suits and .357 Magnums. Every plot point, from the mute girl with curly hair, to the R.A.C.E. invasion, had me hooked.