Demon apocalypse worlds have become such a cool niche lately! The absolute standout for me has to be Brian Keene. His 'The Rising' and its sequel 'City of the Dead' basically defined the modern zombie-demon hybrid apocalypse. The demons aren't just mindless shamblers; they're intelligent, sadistic, and possess corpses. It's brutal and doesn't pull punches. Then there's Mike Shel, though his 'Iconoclasts' series is more of a slow-burn, cosmic horror apocalypse where ancient demons are awakening—less sudden collapse, more creeping dread.
For something with a different flavor, I'd throw in Richard Kadrey's 'Sandman Slim' series. It's not a global apocalypse from page one, but the entire premise is built on the aftermath of a personal apocalypse—the protagonist escaping Hell—and the larger demonic threats constantly pushing the world toward ruin. The tone is gritty and cynical with a dark humor that really works. Jonathan Maberry's 'Rot & Ruin' universe, while more zombie-focused, has some great demonic-adjacent lore in later books that flirts with this territory, especially when dealing with ancient origins.
Ultimately, I think the specialization depends on whether you want the apocalypse as an ongoing event (Keene), a looming background threat (Shel), or the chaotic aftermath (Kadrey). Each brings a distinct flavor of doom.