No doubt about it (and not that I'm particularly recommending it), but the Wizard of Oz. It used to show up on television once a year (I think), so it seemed like a kind of tradition when it would. Somewhere at about Year 30 (estimating) I decided I'd seen it enough.
Second *but distant) runner up: Enchanted
Others in the running (and only because one station went through a time when it seemed to drag out one of these two every other Saturday evening):
My Best Friend's Wedding
And, of course (another favorite of TV stations): It's A Wonderful Life