The Third Man is a very well made, very well cast and very well acted movie.
But that alone is not enough to put it into everybody's top ten movies of all time.
For it also has gravitas, in spades.
It deals, quite seriously, with possibly the ultimate form of evil in our post-Auschwitz world : cold-bloodedly killing innocent little children.
In the movie, character Harry Lime deliberately waters down his already highly profitable black market penicillin ("P" in hospital shorthand) that he is peddling under the table to civilian hospitals in postwar joint powers occupied Vienna.
Even when he knows that watering down the penicillin ensures that babies will die painful deaths from meningitis, when that is actually the easiest of all diseases for penicillin to cure.
That the elusive Harry is so suave and charming, in his few on-camera appearances, only makes the viewers' horror more morally acute.
That 'fungoid growths' could go from the epitome of absolute evil (here reference Hitler,A and Lovecraft,HP) to the epitome of absolute good in only a few short years would certainly make the ghost of Henry Dawson astonished, proud and very happy...