It seems to me that you are setting up a bit of a false comparison when you give examples as diverse such as people who tried to assassinate him, and geopolitical rivals such as Stalin and Hitler. He may have hated all of them, but in rather different ways, and in a different way again from that in which people today may be said to hate Hitler, i.e., as an example of a particularly evil historical figure. There are really at least three different questions here.
The most meaningful and interesting question amongst those that you might be asking seems to me to be the last one: which (to him) historical figure did Hitler despise the most? After all, there is nothing particularly irrational, or certainly nothing uncommon, about hating someone who has tried to murder you, or even someone leading another country in war against yours. Knowing which historical figures Hitler most despised, on the other hand, might give us some insight into his psyche, and why he became so evil.
I do not know the answer, but I should imagine some Jewish and/or Communist leader. Possibly it would have been Marx (who was Jewish, of course) or some other German Communist leader, such as Rosa Luxemburg (also Jewish) or her collaborator, Karl Liebknecht.
Another possibility might be some Zionist leader such as Theodor Herzl, but, inasmuch as Zionism aimed at separating Jews from the rest of European society, in a country of their own, Hitler may well have found it a good deal less objectionable than Communism, which is integrationist (and which he also had other reasons to hate). I should imagine that, to a fervent anti-Semite, having all the Jews out of your country, gathered together in some distant land of their own, where you don’t have to interact with them any more, would be a far from undesirable outcome.