Some great covers mentioned in this and the other thread. I agree that Johnny Cash's Hurt is one of the finest I have ever heard. Special mention also goes to Nirvana's Unplugged covers, especially The Man Who Sold The World.
Anyway, the best cover (which I have eventually worked out based on countless pub discussions) is Joe Cocker's With A Little Help From My Friends. OK, the original was intentionally manufactured to suit Ringo's 'unique' voice so isn't The Beatles finest but still, look what Joe did with it.