The Ubermensch probably has no direct antithesis (I’m just a historian, really), but an ideology like Marxism seems to promote an equality within humanity that flatly contradicts Nietzsche’s philosophy.
Well yeah.
I think all Zarathustra is, ultimately, is the personification of the “Better Living Through Philosophy” concept. Ubermensch means something like “above-man” or “over-man” when translated properly, and I think when translated in such a way gives you a better idea of what Neitzsche intended. The ubermensch is simply the human form Neitzsche gives to his own ideas about what man should be - he is “above man” in the sense that he has evolved, as “schopenhauer” said, beyond conventional morality and thought.
Thus, if he has an antithesis, it is mankind itself, who, in the book, persecute Zarathustra for his ideas that, to them, seem ridiculous because they are not advanced enough to understand or want to understand them.
I think the theological doctrine that man is intrinsically wicked and weak is the antithesis to the Ubermensch.
Theology is going to be a part of history. Overman can be the future. First is the thesis, then the antithesis (correct me if I’m wrong). But that man is intrinsically wicked and weak, could be indeed the antithesis. So to come to the synthesis, we have to know how strongly man is subjected by the ‘dark forces’.
Jeez, I was thinking that the line between black and white is so vanishingly small that its width cannot be distinguished from zero. That there is no gray area.
What did I miss?
It’s just words, De’trop 
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Welcome to the forums Ericac! Nietzsche clearly gives the answer himself in Thus Spoke Zarathustra: part 1: Zarathustra’s prologue where he contrasts the overman with the last man (Walter Kaufmann’s translation. R.J. Hollingdale has it as the Superman vs. the ultimate man.)
Below is an online resource with a translation of the work.
http://users.compaqnet.be/cn127103/Nietzsche_thus_spake_zarathustra/pro3.html
(You will need to advance to the next chapter a couple of times to get the full conversation relevant to your inquiry.
Jesus, there’s a lot of old post bumping these days, no?