It depend on what you like doing while traveling. Christmas is indeed beautiful, I live in NY and I don't feel as if there are too many tourists.
If you like shopping then Christmas and the short period after Christmas is really great. The time after Christmas is probably the best time for concert, show tickets, museums and so on, anything cultural is going to be cheaper and less crowded.
However - it's really really cold! NY is a wonderful city to get to know on foot and if you want to do that, then winter's not the best season at all - try spring or fall or my favorite - Summer!
Whatever season you go to NY in, it's always a fun cool city to visit and I don't think any time is better then other, there are always new and exciting things to do here.