Well I really love to re-play Mass Effect (1&2) and Dragon Age: Origins but I think all of those are too recent to really give a "timeless" stamp XD Most of the games I have are like that... I do like the original Fable, as much as it's a terrible piece of crap in so many regards, but it has REAL EFFING BOSS FIGHTS. TAKE THAT FABLE 2 AND 3!!!
So I'm going to say the first Unreal game. It was the first REAL game we owned. I loved that game, although I've never been able to finish without cheating (my first runthrough ever got about 3/4 of the way through before my sister saved over my save with her cheater-tastic savegame, and I didn't want to start all over again, but I couldn't get rid of the cheats... sob). Also, I love the Navi. I love them so hard. XD