Perhaps, the answer is yes, Reb Yecheskel Landau, the celebrated author of Noda BeYehudah and Rov of Prague, was the most important figure in the history of Chassidus. As we will soon see without him we might not have the Torah of the Baal Shem Tov, Kedushas Levi, Noam Elimelech, Likutei Mohoran, Bnei Yisaschor, Tanya, Sfas Emes and all the other sifrei chassidus.
Reb Yecheskel was a great gaon in Torah and a great tzadik, well versed in the hidden Torah, as well. He was an avid opponent of the newly founded Chassidic movement of the Baal Shem Tov. He was on friendly terms with the Ball Shem Tov, but didn't believe in him as a great tzadik. When the Baal Shem Tov would be in his city, he would stop by to visit R’ Yecheskel, who would personally wait on him, to fulfill the mitzva of hachnosas orchim. When the Baal Shem realized he is causing the Noda BeYehuda to stop learning Torah, he stopped coming. The wife of R' Yecheskel did believe in the Baal Shem Tov as a great tzadik and would visit him with her sons.
In Noda BeYehdah (Yoreh Deah 93) he attacks the practice of saying LeShem Yichud. In the tshuva he quotes a famous posuk with some changes. The posuk in Hoshea 14:10 says כִּי-יְשָׁרִים דַּרְכֵי ד׳ וְצַדִּקִים יֵלְכוּ בָם וּפֹשְׁעִים יִכָּשְׁלוּ בָם, for the ways of Hashem are right, tzadikim walk in them, but sinners stuble in them. He substituted the word Chassidim for פֹשְׁעִים writing "...but chassidim stumble in them". When his grandchildren were reprinting the sefer, many years later, they asked one of the Chassidish Rebbes if they should put the posuk back to its original form. He said to keep it like this because the Noda BeYehudah turned "sinners" into "Chassidim" and you don't want to turn "chassidim" into sinners". (The Chida insists that one should say LeShem Yichud even though there are those opposed to this, referring to the Noda BeYehudah.)
Despite this, he was revered by the chassidic greats, who acknowledged that he meant everything for the sake of Heaven. The Divrei Chaim of Sanz said that if the Noda BeYehuda would be alive he would go sit under his table to learn from him. He is so revered by some Chasidim that on his yahrzeit, the 32nd day of sefira, some purposely do not say the LeShem Yichud before sefira in honor of his yahrzeit, since he has the famous tshuva against saying LeShem Yichud.
R' Yisroel of Rizhin told one of his talmidim, who was a grandson of R' Yecheskel, something he heard over from the Ohev Yisroel, who heard it from the Magid of Zlochov. The Magid of Zlochov said that when it was time for the neshama of the Baal Shem Tov to come down to this world, all kinds of prosecuting angel spoke up arguing that Moshiach was going to come before his time if the Baal Shem Tov comes into the world. Defending angels spoke up in favor of the Baal Shem Tov coming into the world. In the end, it was decided that another lofty neshama will be sent down, who would also be a great tzadik, but an antagonist to the Baal Shem Tov and his teachings. This rift would prevent Moshiach from coming before his time. This other great tzadik was R' Yecheskel Landau, the Noda BeYehudah.
So indeed we can say that the Noda BeYehuda was the most important figure in the history of chassidus, because without him, the Baal Shem Tov would not have been able to come into this world.
The yahrzeit of Reb Yecheskel ben Reb Yehudah HaLevi Landau is this Shabbos, 17 Iyar.
Zchuso Yogen Aleinu