Jolly Boys outing

Porridge and Some Mothers... for me. They actually made decent sitcom episodes first and foremost. Too many Christmas "specials" get too distracted with extended length and the Christmassyness of it all. The best Christmas sitcoms stick to being sitcoms.

It's very difficult to choose but mine would probably have to be the Gavin & Stacey Christmas Special.

Mrs Brown Christmas specials are great too!

Porridge, Some Mothers, Steptoe, WEHTTLikely Lads, Rising Damp, The Good Life all great specials. In Porridge it's the lack of Christmas that works so well, you feel they miss it. And great sitcom plots yes.

Last of the Summer Wine - All Mod Conned, Getting Sam Home, Barry's Christmas, Welcome To Earth
Merry Christmas, Mr. Bean
Father Ted - A Christmassy Ted
Vicar of Dibley - The Christmas Lunch Incident, Winter
Some Mothers Do 'Ave 'Em - Jessica's First Christmas

Couple more I like here they are

Gavin & Stacey Christmas Special

Only Fools & Horses Dates & Time On Our Hands & Heroes & Villans

The Royle Family Joes Crackers & Barbaras old ring.
Benidorm Christmas special 2010

BOTTOM, HOLY, LOVED THIS EPISODE.
BLACKADDER'S CHRISTMAS CAROL.
MEN BEHAVING BADLY, JINGLE BALLS.
LEAGUE OF GENTLEMEN. XMAS SPECIAL.
STEPTOE AND SON, 1973&4 SPECIAL.
VICAR OF DIBLEY, THE CHRISTMAS LUNCH INCIDENT.
STILL GAME, ANY SPECIAL.

I agree with what some others have said, Gavin and Stacey is a great one, and scarily accurate of what an Essex family is like at Christmas...
Miranda, Father Ted and Porridge also spring to mind.