Super Smash Brothers... Bought it the month it was released, still have the lovely box and manual. This game really left an impression on me, I found it to be very imaginative, creative, and enjoyable. To this day I still pop this game into the N64 every now and then, despite that I can't get anyone to play N64 games anymore. It is quite a bit different than Melee and especially Brawl, it is like the purest form of Smash and would be the best if it had more content.
I got pretty far into Donkey Kong 64 but got to a point where I just couldn't stand it anymore. It's a real shame just how much the abundance of collecting ruins the game, because it would be an ABSOLUTELY AWESOMETASTIC game otherwise. A game which I'd have probably played through a few times by now has become a game I switch off in disgust.