Donkey Kong 64, but Donkey Kong Country 2 comes pretty close. I never really played DK64 as a kid for some reason, but after giving it another shot a few years ago, I fell in love with it. After playing Banjo-Kazooie and Tooie nonstop for 10 years, I guess it was a very fresh experience for me. The collectables aren't too big of a problem, but it is
pretty annoying how you have to backtrack to a Kong barrel to try and get some bananas in an area that's meant for another Kong. I actually played through it again a couple months ago and 101%'d it.
I like Donkey Kong Country 2 a lot as well, mostly because of how artsy it is. The soundtrack for DK64 is great, but the soundtrack for DKC2 is just a masterpiece. The graphics are fantastic and have a style that no other series has yet to replicate, and the level and world designs are wonderfully crafted.