What is your favorite cover?

Probably this.

Quicksilver Messenger Service - Happy Trails (Roy Rogers & Dale Evans)

Tomita - Pictures at an Exhibition (Mussosky/Various Artists)

The Force - Star Wars Disco (John Williams)

Manfred Mann - Blinded by the Light (Bruce Springsteen)

Honorable Mention:

Fraternity of Man - Don't Bogart that Joint (Okay, maybe there wasn't an earlier version, but the chances of these lyrics being sang, hummed, slurred, or st-st-st-studdered by the Flower People in the Bay Area after a couple of "hits" in the 60's was a certainty)

Doobie Brothers - Black Water (First sang by Bob Winnebago in 1966 as he was dumping the sewage tank on his first RV)

Diana Krall's spin on the stunningly beautiful Joni Mitchell tune A Case Of You. 

I love covers. Thanks for this post so I can find some new gems! I like so many, but one I always return to is Shawn Colvin’s cover of "Some Day" by Steve Earle. But my latest love is a cover of an early Elton John ballad, "Love Song" by Melody Gardot — an audiophile treat to boot.

And to follow up on @bigtwin 's comment, it is interesting to consider what makes a good cover. So much of music history had a separation of composer and performer. I always think of a good cover as one I want to listen to as much or more than the original. Sometimes a new performance shows something else magical about the composition itself that the original doesn't capture for lots of reasons. Thanks again to the OP for generating this offering of performances!