It is the most covered Christmas song of all time (that’s a fact, not just a phrase). Most lists would probably have it higher, but there is just too many other good stuff crowding it out. It’s legendary now, and was co-written by Mel Torme, a crooner who has many a classic Christmas song under his belt.
But, Mel doesn’t have the best version of this. That belongs to the soulful dynamo…
The Christmas Song – Nat King Cole
ThirtySomething 
To me, this is the only version of the song. Although Johnny Mathis gives it a respectable try.
This is a favorite every year in our household. We always listen a Nat King Cole album while we decorate the tree. Good times.