For me it's the obvious choice of the gang declaring, "Merry Christmas, Charlie Brown!", followed by (most of) Hark the Herald Angles Sing as the snow starts falling (and now with Peter Robbins', Christopher Shea's, and Tracy Stratford's names added to the credits today). It just helps sum up the overall true meaning of Christmas the show sought to find perfectly. Honorable mention:
-Rudolph's triumphant flight through the clouds as (at least since 1965) the Misfit Toys drop down to their new homes (although note that the elf pushes the bird that can't fly off the sleigh without an umbrella....)
-Frosty leads his parade through town once more, then takes off while declaring he'll be back on Christmas Day
-Emmet Otter's Jug Band Christmas: Emmet, Ma, and the band jam it out one last time on the river while the camera pulls up for one final impressive nighttime shot of Frogtown Hollow
-Santa Claus Is Coming to Town: S.D. Kluger essentially leads a cast curtain call on the road to Santa's castle to the main theme, with the man in red himself taking the final bow in closeup, as it were
-Christmas Eve on Sesame Street: Big Bird, now found again and realizing he doesn't need to see how Santa gets down the chimneys to enjoy the season, wishes everyone else on the Street a merry Christmas to the reprise of Keep Christmas with You All Through the Year, finishing in front of the Street's main Christmas tree as the snow falls--followed by the closing joke of Cookie eating Gordon and Susan's Christmas tree
-Twas the Night Before Christmas: one final tour of Junctionville, now all lit up and happy once more, over the instrumental Christmas Chimes Are Calling Santa reprise
-Yes Virginia, There Is a Santa Claus (1974): Jimmy Osmond performs the title theme over scenes of 1890s New York at the holiday, followed by a pullback to the original shot of earth from space seen at the beginning
-Claymation Christmas Special: "Say good night, Herb." "Good night, Herb." Followed by the cast closing with We Wish You a Merry Christmas, and Herb getting the last word in