Half Life 2 has aged extremely well. Even to this day, I consider the graphics "next gen", and it propelled the idea of a living breathing world with interesting and sympathetic characters. Oh, and the gravity gun was an awesome creation.
My friends and I still play the old C&C's and Starcraft, and for games that were for the most part, pretty old, we all agree that they are still great.
Smash Bros. too aged pretty well. Yeah, novelty of fighting Nintendo characters wore off but it was still fun as hell, and at the end of the day, isn't that what matters?
Aged horribly? Halo 2 and the last generation Grand Theft Auto's. Halo 2 was great because it had a multiplayer component (online), but now when you look back at them, it was without a doubt the worst. It felt like Halo, it WAS Halo, but it was not the stellar experience that was Halo 1 (well for me anyway. But that was my first FPS game ever, so yeah.

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The GTA 3 series aged badly too. I still play them, but the graphics were n64 era, and the controls didn't sit right when compared to GTA IV.