Mike, I'll try and remember to check it out later, as I can't at the moment, and I'll try to give you some feedback too.
For piano covers, as in songs played completely on piano, not accompaning your own singing with the piano, I'd say some of the most covered songs would have to be (on YouTube, that is) OneRepublic - Apologize, Vanessa Carlton - A Thousand Miles, Evanescence - My Immortal, Coldplay - The Scientist. Basically, any popular hits will be covered a ton on YouTube, especially, if there's really a piano involved in the original, as in these three, and it's also a big part of the song. Some older popular piano music for coverers would be the hits of Elton John and The Beatles, and probably Desperado by The Eagles, too.
For guitar, covers are usually either challenging solos by electric guitar, or acoustic guitar and singing. For the latter, some popular songs are Wonderwall, Save Tonight, Hallelujah, I'm Yours.
Basically, most songs that are covered by many people have a recognizable riff in it for piano or guitar, and are known by everybody. That makes it easy to get views, as people search for their favorite pop songs on YouTube, and if the original consists of the instrument you play it in, they might be more interested in hearing your take on the song, and they will recognize the tune better.