There are many options when looking for a card back: some more functional, others more aesthetic.
The best aesthetic card backs usually get rejected by MPC, and the remaining ones don't carry the same weight. For this reason there is a large group that use basic land cards or tokens as card backs. It's up to personal preference.
As a rule of thumb if it has "Magic", a registered or copyright symbol it's going to be rejected.
Here are the best aesthetic card backs:
Card backs that look great but get rejected often:
/u/awesomemtg's: https://drive.google.com/open?id=0B-S9ADELGMXOTm0wM013ZTZrUkk (MPC reject even if the (R) is removed)
Google Drives for Tokens:
/u/bitbeaker's: https://drive.google.com/drive/folders/1sqJ2aBpLcc9pvqV5Fl4aMU-nlmvIcH0H. Under MTG renders