Hmmm...maybe i'd say Jimi Hendrix - All Along The Watchtower
...but being a big 'Fleetwood Mac' fan id also say "Black Magic Woman" - Santana / Fleetwood Mac
You all should check out www.secondhanRABongs.com . Its a database devoted to song covers, good for figuring out who has covered a particular song, when, and who wrote the original, how many times a song has been sampled, what songs the sample it appears in, and a whole bunch more information regarding covers. Great resource!
Patrick Wolfs covers of Afraid and Ari's Song both originally by Nico are beautiful. Granted Nico sucked to begin with so its not that hard to top the originals, but he made them amazing songs.
Jeeez you guys arent very open minded, just be cause Noel is an ass and Chris Martin is in Coldplay doesn't make it a bad cover. Because 1. Chris is a good singer and 2. Noel & Jonny Buckland are good guitar players.