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.