Third Most Covered Artist of All Time: Neil Young
Neil Young is considered to be the third most covered artist of all time, at least as ranked by the article "Purple Words on a Grey Background: Four Decades of Neil Young Covers" by Phil Dellio in Stylus Magazine. Only The Beatles and Bob Dylan are ahead of Neil of other artists covering their songs.
The article does a pretty comprehensive job laying out artists that have covered the classics as well as the obscurities.
Devo's "Ohio" has always been a personal favorite.