Covers are almost always worse than the original. Exceptions welcome.

[FrostFyre]

I like Pentatonix a lot

[RagnerTheHiker]

I enjoy some covers as well. Especially instrumental ones. 

[LeonPandora]

My favorite cover is Under Pressure, covered by Marc Martel. The original was released by Queen and Davide Bowie. While I don't think it's better than the original, it's a very enjoyable song.

I also recommend Kfir Ochaion's guitar covers.

[caldella]

The Cure's lead was hesitant about letting Johnny Cash cover Hurt but felt afterward that he did the song even more justice. I love Dolly Parton, but Whitney Houston's version of I Will Always Love You is more famous with pretty mind blowing vocals. Even Dolly said it blew her away so much she had to pull over her car when it came on the radio. I'm also a bigger fan of Eurielle's Song to the Siren vs the original Tim Buckley version.