You know it's funny. when ppl ask me this I think they expect a slightly more obscure or recherche pick like one of the Frontier elders or etc., but the reality is: I adore the Deviljho. Thematically, visually, as a hunting experience, every element of it has consistently spoken to me ever since I first encountered it back in 3rd Gen on release. I like it even more when it's paired with the Rajang, the other contender for 1st place in my estimations. Back when I was a kid on 06/7 YT looking at Rajang footage from the unlocalized MHDos I was losing my mind - the conceit of a creature that embodied all mammalian traits and stood at the apex was such a massive contrast to everything that had gone previously, and i've always loved how it recalls both a sort of wrathful thunder deity and a gelada baboon.
Honorable mentions to - Astalos, Mizutsune, Garuba Daora, White Fatalis, Gogmazios, Azure/Pink Raths, Lavasioth, Cephadrome, Gore/Shagaru Magala and the Yama Tsukami, among many many others. it's harder to find a monster I don't like, I think almost all of them have something to offer!