My friends and I used to argue about this for hours while snacking on instant ramen, but here's my take: the most common pairings in 'Obey Me!' fanfiction are split between each demon with the human MC and lots of inter-brother ships. On the MC front, Lucifer, Mammon, Leviathan, Satan, Asmodeus, Beelzebub, and Belphegor each have massive followings — Lucifer x MC tends to get the regal, slow-burn romance treatment, Mammon x MC is jealous-fluff or possessive-protection, Levi x MC is awkward-adorable nerdy comfort, Satan x MC is bibliophile intimacy, Asmo x MC is glamorous, Beel x MC is food-and-cuddles, and Belphie x MC is sleepy, cozy slow-burn.
Among the brothers themselves, people adore enemies-to-lovers and trope flips: Lucifer/Mammon shows that stiff-upper-lip vs chaotic sibling energy; Levi/Satan is a nerdy, introvert-y comfort ship that’s low-key wholesome; Mammon/Asmodeus gets wild, chaotic chemistry; Beel/Belphie is the sleepy, food-loving domestic pair. There are also lots of poly ships and found-family stories, and smaller pairings like Lucifer/Satan or Levi/Mammon flourish in angsty fics. If you like tropes, search for fluff, angst, domestic, and rivals-to-lovers — that’s where the fandom’s heart lives. Personally I gravitate toward quiet, candlelit Levi/Satan reads, but I’ll stan a silly Mammon/Asmo bromance any day.