The one who skinned a child.

The one who betrayed legendary heroes and the one that kidnapped aliens youth

While most of them are vile enough but I got to go with Reclusa cause not only he destroyed multiple worlds but literally put people in simulation, trapping them untill they starved or thrist to death its kind of like a fate worse then death.

Sentinel hands down. Bro slaughtered the Primes, who btw trusted him fully all to usurp Cybertron for himself which even then wasn’t fully his as he was just a puppet, and crippled a whole generation of Cybertronians, something so morally horrible everyone was in denial when they heard, and make them slaves that mined their life force for their enemies, all to keep that joke of a power.