Was it an open or a secret vote? I think that's very relevant to the op's question.
Ok, so as far as I can tell the vote was anonymous, thus the problem is that we might never know who did not vote for Hitler in that meeting.
When reviewing the fragmentary early membership lists of the NSDAP, there are a few possibilities who could have voted against Hitler but the most likely is Anton Drexler.
Drexler was one of the founders of the NSDAP (then named Deutsche Arbeiter Partei, German Workers' Party, DAP) in 1919 and Hitler's main rival for party chairman in 1921. Drexler who had been the second chair of the party had taken over first chair in 1920 when his predecessor Karl Harrer resigned over conflicts with Hitler. A similar fate awaited Drexler, who was unable to mount any serious resistance to Hitler. Hitler had parctically become indispensable for the party by 1921 because of his success as an orator and he guarded that role as well as what he understood as ideological purity of the movement jealously.
In 1920 and 21 the NSDAP was plagued by a shortage of money and by being one among the many völkisch movements in Germany. To alleviate both of these problems, they attempted to merge with other, similar, parties such as the DSP and the Deutsche Werksgemeinschaft. Hitler opposed this because it threatened his supremacy in the party, so he used his own role as leverage. Successfully crashing the DSP negotiations, he attempted the same in connection with the DWG negations but failed and so, in a fit of rage, he threatened to resign several times in 1920 and 1921. Every time, fearing the loss of their popular orator and really, the only thing that set them apart form the dime a dozen völisch parties in Germany, the NSDAP leadership placated Hitler by giving him what he wanted.
Again sent to to convince Hitler not to resign, Drexler asked him in July 1921 what he wanted. Hitler demanded chairmanship and extraordinary powers within the party. While Drexler and Hitler showed their unity on July 1926, Drexler as the current chairman was obviously the choice for those who opposed Hitler within the party and there seem to have been at least some because according to Ian Kershaw, 3000 anonymous pamphlets attacking Hitler had been prepared by the time, the members' meeting rolled around.
So, it might have been Drexler who voted against Hitler, possibly expecting a victory of himself, which obviously didn't pan out. Drexler left the NSDAP when it was dissolved after the Hitler Putsch and only rejoined in 1933 never again to gain any kind of political role until his death in 1942.
Other possibilities include Gottfired Feder, prominent member of the anti-Capitalist movement within the NSDAP, who later served in minor roles in the Nazi adminsitration; the first chairman, Harrer, though it is unknown if he was even there (he died of natural causes in 1926); or one of the lesser known early members about whom little information can be found such as Hans Baumann or Karl Beggel; or it could have been Hitler himself following convention in some parties of abstaining to vote for himself though given his personality that seems rather unlikely. In the end, it probably is not possible to find out with any kind of certainty.
Edit: Apparently, Robert Payne claims that the dissenting vote came from a man called Rudolf Posch, a party member and librarian (who is not to be confused with the Austrian Social Democrat of the same name). I have no idea where this claim comes from. Neither Payne nor Joseph Howard Tyson who also mentions this in his book on Dietrich Eckhart have that fact footnoted, though it is likely that one of them got that fact from the other. I could go and look up his Nazi party file though according to the information in the search engine of the German Bundesarchiv it is rather thin and it seems that man never made any sort of impact on the history of the NSDAP again and what became of him is unknown.
2nd edit: for more on the conflict between Hitler and others in the party see /u/boffcheese comment here
Sources:
Ian Kershaw: Hitler.
Wolfgang Horn: Der Marsch zur Machtergreifung. Die NSDAP bis 1933.
Mathias Rösch: Die Münchner NSDAP 1925–1933. Eine Untersuchung zur inneren Struktur der NSDAP in der Weimarer Republik.
Ludolf Herbst: Hitlers Charisma: Die Erfindung eines deutschen Messias.
Were you allowed to vote for yourself? Or is it known for definite that there was someone voting against the grain?
Could you show me a source for the claim in your question, as far as I know the vote was unanimous, if you're refering to the 29 Juli 1921 vote for the function of Führer der NSDAP ?
German quote:
"Doch erst am 29. Juli 1921 fand im Cirkus Krone eine außerordentliche Mitgliederversammlung statt, in der Hitler mit 554 abgegebenen Stimmen einstimmig zum Parteivorsitzenden (Führer der NSDAP) gewählt wurde. Die NSDAP wurde nun völlig auf die Person Hitlers ausgerichtet und dessen Vorstellungen vom Nationalsozialismus für diese als verbindlich eingeführt."
source: Joachim C. Fest: Hitler. Eine Biographie, Verlag Ullstein 1973.
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