The oldest would be all of my N64 games, so Banjo-Kazooie, Super Mario 64 and Zelda: Ocarina of Time because I got them all at the same time.
(I got given my N64 by my cousin when I was a kid, and those just happened to be the only games he had for it, lucky me)
Newest? Monster Hunter Tri for the Wii.
I bought that today and have only played an hour of it (in solo, too) so I can't really compare yet, plus I never finished any of my N64 games due to being a nooby kid at the time, I don't really fancy going back and completing them, it wouldn't be the same as if I was still a kid anyway.