Starting off the list with a pretty well known name, but still deserving of recommendation for his talents, we have the acapella extraordinaire Smooth McGroove.
There are a lot of great acapella artists on YouTube, and Smooth isn't even the only one on this list, but he is arguably the best of them (on this list), and unarguably the most prolific.
Smooth does covers of all sorts of classic video game tracks and he manages to recreate these songs using only his voice over the course of several takes, all playing at once without any instrumental accompaniment (a one man acapella). His "baas", "buhs", "deets", and "doos" are all he needs and he manages to make impressively catchy renditions of classics.
Some people have trouble even hearing the original version of song that Smooth has covered because all they can hear, in their head, is his version. "Gaur Plains" from Xenoblade Chronicles and "Forest Maze" from Super Mario RPG will never sound quite the same again after you've heard them from him.
Smooth McGroove, his name says it all, and you can check out his bandcamp here.