8. Joan Baez in "Mary Hamilton (Bob Dylan version)"
7. Paul Simon in "A Simple Desultory Philippic" by Simon and Garfunkel
[no embedding on this one, sorry]
6. Weird Al Yankovic in "Bob" by Weird Al Yankovic
5. Hugh Laurie in A Bit of Fry and Laurie
[no embedding on this one either]
4. Ed Mann in "Flakes" by Frank Zappa
(At 1:19 in the video)
Note: the studio version of this one was done by Adrian Belew, an impression I would have ranked slightly higher than Mann's. This one's great, too, though.
3. John C. Reilly in Walk Hard
2. John Lennon in an unreleased solo track
1. Mark Knopfler in his entire life