My money would be firmly on communism - fascism in Germany would be pretty untenable for several reasons (lack of a unifying religion; strong left-wing movements since the 1880s; reluctance to go back to right-wing autocracy in the style of the Wilhelm II years), whilst communism fits a lot better with the national psyche. Marx and Engels, after all, were German, and the ideas of Ferdinand Lasalle would have united nationalism and socialism into a potentially very strong movement; meanwhile, the Lutherans and Calvinists of the north wouldn't have too much trouble with an ideology preaching hard work in the service of a greater good. It's worth remembering that there were several strong leftist movements in the early 1920s; the Spartakists tried to take over in 1919 and were only stopped by a combination of Freikorps regiments and figures in government like Drexler (if I remember rightly, that was the event that caused him to rise to the political mainstream), whilst the secessionists in Silesia and Bavaria and the Los von Preussen movement in Hanover were other sources of discord. The SDP even managed to enter government under Bauer in 1927 - a sign of how comfortable the German people were with socialism. Fascism, meanwhile - well, it never really took off in Germany at all, but there was that populist ultraconservative group led by Rohm that tried to instigate a military coup in 1925. The North Swabian Democratic Alliance for Prosperity, I think they were called. They never really got anywhere - nobody wanted any of their policies, and their relentless attacks on the rest of the West were very unpopular.