I have been tricked OTL I think I can agree that mine change depending on my mood or what fits with what I want to see in MRM at any given point lol, but let me try to name the most persistent ones:

-For Mob, definitely a praise kink. He's elated at seeing Reigen being proud of him, and would maybe be a little too serious in following his advice/instructions in bed lol. I also think he might have a thing for being ordered around a little hehe. Probably he could develop a thing for those sock suspenders and ties, from seeing Reigen wear them so often. Not sure if it counts as a kink, but Mob being so into seeing Reigen sucking him off... In any case, I think I have a much more difficult time thinking of Mob's kinks in particular, it really depends on the mood/au.

-For Reigen, he definitely has a degradation kink. How intense depends on my mood, but I feel like him trying so hard to give off an appearance of being a functioning adult in control of his own life might make him enjoy Mob being a little mean to him here and there. Also he 100% has an oral kink of some sort. He can't stop putting things into his mouth in canon, he so makes a whole feast out of sucking Mob off. Also ever since the omake where he chokes on a rock thing and people started drawing fanart of Mob sticking his fingers into his throat to lodge it out my brain chemistry was altered forever.

AH, also, for both of them: they have a kink related to pretending they're still shishou and deshi even once they're both older. I personally want Mob to have a kink of blowing Reigen under his desk while they pretend like they're still 2814-ish, so there's that too