My favorite au would definitely have to be the mafia au. I love the designs so much and it actually plays well with both dynamics (mobrei/reimob). I personally love the hc of Reigen taking in Mob (and Ritsu) at a young age and essentially raising them which leads to Mob being utterly devoted to Reigen. He wears Reigen's jacket no matter how old he gets, even when it inevitably doesn't fit around his shoulders like it used to. He would die for him, he would kill for him. Mob would watch the whole word burn for Reigen and that makes me just bfjxjjdbfjs
Anyway, yeah. I love me some mafia au