I recently saw Derek Kisman's (aka Snapdragon) Donkey Kong Country video, in which he beat the game with 101% completion in 50 minutes with no deaths, and also his Donkey Kong Country 2 video, in which he beat the game with 102% completion in 1 hour 33 minutes with 1 death (though it didn't look like the enemy should have hit him). Both were done without saving the game at all. He blasts through each level with amazing speed and accuracy, and very rarely messes up. This guy has awesome skills, and must also have a really good memory to be able to remember exactly what movements to make over such a long time period. These are probably the most impressive video game achievements I've ever seen, so if you haven't seen them yet, I strongly recommend that you see them soon. Even though each one is a few hundred MB it's definitely worth the wait.
Edit: Oops, I forgot to post the site where you can download these videos from. Here it is:
http://planetquake.com/sda/other/other.html