Saint Vladimir! He fucked up slavic paganism until it fell apart, and then converted the Kievan Rus’ to (orthodox) Christianity. According to legend, he interviewed a bunch of religions before settling on Orthodoxy, including (German) Catholicism, (Volga Bulgarian) Islam, and (Khazar) Judaism.

ETA: He told the Khazars “well, clearly your God isn’t the most powerful because you lost the temple,” and he told the Volga Bulgars that he couldn’t convert to Islam because “drink is the great pleasure of the Slavs,” and he couldn’t deny it to them. As to German Catholicism, he apparently just found it depressing.