The "songs you don't like" thread had something in there which I had to stop myself from replying to as was off topic but then thought deserves a thread of its own.
Red Red wine by UB40 was put forward as a "don't like" which I have to agree was awful (UB40 were a popular band at the time) and I think it was No. 1 in the "hit parade" for a while but nowhere near as good as Neil Diamond's.

Conversely, there were songs I grew up with and only later found out that they were covers, some better than their originals.
One I immediately think of is State of Independence by Donna Summer.
I loved the original by Jon and Vangellis but I the late Donna's when I first heard it blew the original out of the water.

Any others?