Candidates would certainly include Knockin' on Heavens Door, I Shall be Released and Don't Think Twice it's All Right, but I think a strong contender might actually be one of his newer songs. Make You Feel My Love off of his 1998 album Time Out of Mind.
I have 5 versions myself on my iPod, in no particular order: Dylan Bryan Ferry Trisha Yearwood Garth Brooks Adele
And the Wikipedia lists Garth Brooks, Trisha Yearwood, Billy Joel, Neil Diamond, Joan Osborne, Luka Bloom, Josh Kelley, Timothy B. Schmit, Kelly Clarkson, Ronan Keating, Mick McAuley and Winifred Horan (from Solas), Emily Loizeau, Bryan Ferry, Mary Black, Jasper Steverlinck, Ruarri Joseph, Phil Keaggy, Taylor Hicks, Adele, and Kris Allen.
And more good Dylan news. His next album ships end of April.
Slate has a very nice article and slideshow on Robert Frank's The Americans. It is definitely one of the most important works of 20th century photography and it is nice to see it revisited.