A Charlie Brown Christmas has always been, and will always be my favorite animated christmas special. While it lacks the higher quality animation of more recent specials and doesn't have a particularly extravagent plot, it's heart and emotion is really unmatched in my eyes. The fact that it also is one of the few specials to use the original, religious meaning of Christmas in a way that isn't "in your face" is also welcoming, and I'm glad that it still is shown regularly on the holidays, within a climate that often is far less welcoming of non-secular media.
A close second would be How The Grinch Stole Christmas. Easily the best interpretation of any of Dr. Seuss's classic tales ever, and it's original songs are fantastic. Like Charlie Brown, it may not be technologically impressive by modern standards, but it's warmth and message make it stand out as a true classic.