There were a fair number of right wing nationalist parties in Germany in the 1920s and 1940s that resembled the NSDAP in both rhetoric and tactics. There were also far left wing parties like the communist KPD which actually resembled the Nazis far more than either side would have liked to admit. Many parties had their own paramilitary forces like the Brown Coats, and there were other additional unaffiliated paramilitary groups who at times allied with one party or another. So the Nazi's weren't particularly unique in what they were saying or what they were doing.

Had Hitler not taken control, the next likely candidate would have been the communist KPD. They took third in the 1930s elections and were fairly popular. So they might have been able to pull off a Nazi style coup.

Of course communism wasn't as popular as right wing nationalism as a whole. So had the Nazis not existed, it's easy to imagine that their supporters would have rallied around the DNVP or the DVP. In fact most Nazi growth came from these two parties. The DNVP was also the party that spawned the Nazis, and their coalition support was critical in getting Hitler the Chancellorship. The DNVP might not have started WWII if they got in power. But they officially denounced Versailles, promoted the creation of a pan-Germanic state, were anti-Semitic. So a DNVP dominated Germany likely wouldn't have been too different from a Hitler controlled one.

Most historians nowadays would probably agree that the KPD did not stand a great chance of seizing power in 1933. Compared to the Nazis, they had all of the same enemies in the center of the political spectrum and none of the allies on the right. Recall, too, that they had already once tried to seize power in 1918-1919. Even in its weakened state, the German army and ruling socialists found the means of crushing the revolt.

As for the runner up in 1933, apart from whether or not that runner up was fanatical or not, I would say it was either Franz von Papen, Kurt Schleicher, or another conservative or military-backed candidate. At the time, President Hindenburg still had the power to make or break Chancellors. Papen, who had served as Chancellor in 1932 after Heinrich Bruning, was Hindenburg's favorite but did not enjoy the backing of the army; Schleicher was an army man, but during his brief reign in 1932-33 had proven himself unable to break the Nazi movement.

Had Hitler lost the confidence of the army and they had attempted a coup (as many of them had in the 1920 Kapp Putsch) say in 1933 or 1934, they probably would have put a trusted officer at the helm (Von Seeckt, head of the Reichswehr in the early 20's, was discussed in some circles at different points), and ultimately would have in all likelihood sought the restoration of the Kaiser, still alive and well at the time.

As long as Hindenburg was still alive, Papen was always a viable choice. In 1934, Hindenburg charged him with forcing the Nazis to tone down their revolutionary antics, especially the violence of the SA, but he ultimately failed, prompting Hitler to crack down on the SA to ensure he would retain the trust of the conservative military old order. After that time (June, 1934) and especially after the death of Hindenburg in August 1934, it became exceedingly unlikely that anyone would be able to overtake Hitler.

The military as well as the traditional conservatives - Papen was a Catholic Centre party member, and the DNVP certainly did not have a monopoly on conservatism - wanted to return Germany to its pre-1918 system, with a Hohenzollern or at least a monarch of some sort, and I think it more than likely that Schleicher and Papen both fancied themselves Bismarck's heirs. They might have made moves to restore German greatness. It is exceedingly unlikely, in this redditor's opinion, that they would have embarked on war on the same scale as Hitler.

I've written recently about related issues:

[On the clubs and militias that sprung up around Germany in 1918] (https://www.reddit.com/r/AskHistorians/comments/4gqtjr/what_was_the_significance_of_the_new_political/)

On the German military's involvement in Hitler's rise to power

While I am not sure if the "fanatical" tag would apply. Could a subject matter expert explain How close communist and socialist were to coming to power instead of Hitler?

Were there more Mainstream Socialists as well as Fanatical Agitators fighting the Nazis in the street?

What were the major groups and how did their positions differ from each other.

Were there any Communist gradualist?