Wozza said:
Gen. Lettow-Vorbeck would receive broad support from the center and right wing of German politics. These as the same people that supported Hitler. If Lettow-Vorbeck is successful as chancellor I would expect to see the Nazi party melt away. Only a tiny, inconsequential Nazi party will remain.
Why? Why is his position different to that of Schleicher or Papen? He has no mass party or street presence.
From First World War.com, "Returning to Germany as a national hero (and having been promoted general in the field), Lettow-Vorbeck was likewise admired by his former enemies as a courageous, tenacious and honourable fighter. Once in Germany he immediately joined the Freikorps, and at the head of a brigade successfully crushed Spartacist forces in Hamburg....From May 1929 until July 1930 he served as a deputy in the Reichstag, later unsuccessfully trying to establish a conservative opposition to Hitler."
So he did have a "street presence" in the early days. He actually served in the Reichstag briefly in 1929-30, and he opposed Hitler strongly, something Papen didn't do (Papen lifted the ban on the SA and advised Hindenburg to name Hitler Chancellor!).
Schleicher actually wasn't too bad, but he was a behind the scenes man who rose to power through Hindenburg, but was also removed from power by Hindenburg. He was by comparison to von Papen a more moderate conservative, authoritarian but with more parliamentary leanings. He was opposed to Hitler, but was willing to work with Strasser, so a little weak on the Nazi front. However, I see nothing to indicate a similar level of popularity among the public or the military comparable to von Lettow-Vorbeck. In fact, Schleicher was murdered in the Night of the Long Knives, whereas Lettow-Vorbeck was not. Quite frankly, the reasons Lettow-Vorbeck survived was probably because he wasn't in power, but also because he was still a national hero, the only undefeated commander of WWI, and it would have turned the Army against Hitler had he been assassinated. Apparently, the same was not true of Schleicher.
If von Lettow-Vorbeck had not supported the Kapp Putsch, and instead had stayed in the Reichswehr, then if Hindenburg had elevated him to the Chancellory instead of Schleicher, there would have been a chance that the Nazis could have been opposed, and quite frankly, Lettow-Vorbeck was not one to follow orders he believed counter to nation and honor, and if Hindenburg had tried to pull on him the stunts he did Schleicher, I could see Lettow-Vorbeck attempting and even succeeding in a coup at that point. However, there would be a lot of fighting, and it is not entirely clear to me who would win in the end, the Nazis, the anti-Nazi Right, or the Socialists.
So Lettow-Vorbeck is of a different quality than the others you mention here. In a different constellation of circumstances, it is entirely possible that he might have come to power, and even thwarted both the Nazis and the Socialists/Communists. However, I am not certain that he would have been the one to save the Weimar Republic. We have no evidence historically of his abilities in Governance (though he seems a governmental genius in Alternate History governance). And he was much more of a right wing, monarchist than anything else.
What distinguishes him from most others of his time was his successful war record, his clearly non-racist views, and his ardent anti-Nazism AND anti-Communism. These features are what make him such an alluring figure for Alternate Histories.
Practically speaking, at least from my cursory reading of the Weimar period, Stresemann seems to have a much more substantial record in terms of actual policies of benefit to the Weimar Republic, as well as enough political clout to have them implemented. However, he was fatally weak in the Beer Hall Putsch, and that isn't promising. I wonder if there wouldn't have been some way to team up Strasemann and Lettow-Vorbeck, allowing the two bolster each other's strengths....