Mine’s got to be the muppets version, when I watch that film and hear the songs I know it’s christmas! But as much as I love all the different film interpretations, for me nothing beats the book!
What an odd thread to read on a 100 degree day! 
Alistair Sim owns the role for me, no question. The whole production is so Dickensian, without adding any pseudo Victorian fluff, and it’s genuinely scary. Helps that despite its having been made in 1951, it looks like a film made twenty years earlier – darkly lit, sharp distinctions between black and white, with fairly sparse set designs. My God, the gruesome undertaker/charwoman/laundress scene in that grubby workhouse…! It’s a work of art. And the character actors filling the cast are golden. Marley, Cratchit, Mrs. Cratchit (and all the small assorted Cratchits), Fezziwig, the ghosts, Mrs. Dilber (the maid), the undertaker, even Fred’s gormless friend Mr. Tupper and the little maid who silenly encourages Scrooge to go in and join Fred’s party (and who looks eerily like a young Audrey Hepburn)… they’re brilliant.
The extra parts don’t bother me – heck, even though Mr. Jorking (the embezzler) is a completely made-up character, he’s one of the most Dickensy cast members in it!
I have to disagree with the notion that Sim’s Scrooge is nasty and mean without any reason for it. I think you can see glimpses of the lonely, hurt old man inside those expressive eyes right at the start. Especially in the scene between him and Fred, when his nephew invites him to dinner. Scrooge casts this sharp look at the boy and says, “Why did you marry against my wishes?” and the line isn’t just bitter and overbearing… there’s hurt and disappointment there too. In the flashback scenes you can see the genesis of the man he became, not just in the obvious death-of-his-sister scene but in the death-of-Marley scene, where he’s almost apologetic despite his hardness, and realizes what he’s become (emphasis mine): “You mustn’t reproach yourself, Jacob. We did no worse than anyone else. Or no better, if it comes to that.”
BTW:
That would be a good trick, because Sim doesn’t say that line. 
Unless I’m very much mistaken, and more mistaken than someone who’s watched the film every year for thirty-five years is likely to be, it’s not in the 1951 film.