Ambersons all the way for me. In its truncated form, it often makes "10 Best" lists. In its uncut form, I would not be surprised to see it edge out "Kane" for the top spot.
I agree with Jeff that it would be great to see an uncut Welles film made under optimum conditions. No matter how much I love Othello, for instance, there is no question that the film is, as Jack Jorgens describes it in Focus on Orson Welles, a "flawed masterpiece". The problems in adding a soundtrack, matching shots from different years and continents, the need to have the actor who is speaking to turn away from the camera, etc led Othello, great as it is, to be flawed. Likewise, Chimes at Midnight, masterpiece that I feel it is, suffers from rather indistinct muddy sound (something that could perhaps be "fixed" by engineers if any money could be spent to restore it). I think TOSOTW would have been another of the more "ragged" Welles productions along the lines of Othello, with a changing cast, story, etc. Also, being a color film, it would likely be very discolored today, as many films from that era are, and since it would have been an OW film, you can be sure tha not a finger would have been lifted to preserve the film.
TOSOTW would interest me in that it would be a "new" Welles project, but an Ambersons cut restored from its 89 or so minute form to it 130 minute plus form would be like a "new" film as well.
Natalie: Orson wept when seeing Ambersons because it had been butchered. The best way to dry Orson's tears, or at least turn them to tears of joy, would be to find the missing footage and restore Ambersons. I'd cry if I saw it uncut, too!