Which Autobot becomes the new leader after Optimus Prime's death in the 1986 film?

After Optimus Prime dies in the 1986 film, Hot Rod becomes Rodimus Prime and takes over as Autobot leader.

The transition occurs during The Transformers: The Movie, set in 2005. Optimus Prime is mortally wounded while defending Autobot City and gives the Matrix of Leadership to Ultra Magnus. Magnus cannot fully unlock its power, and the Matrix is later taken by Hot Rod during his battle with Galvatron inside Unicron.

The Matrix transforms Hot Rod into Rodimus Prime, physically enlarging him and granting him the authority and power associated with the Autobot leader. Rodimus then opens the Matrix inside Unicron, destroying the planet-eating Transformer. The film therefore links Optimus Prime’s death, Hot Rod’s transformation and Unicron’s defeat in one succession sequence.

Ultra Magnus is the main source of confusion: he briefly carries the Matrix and acts as commander after Optimus falls, but he is not the film’s eventual successor. Rodimus Prime later leads the Autobots through much of the third season, although Optimus is eventually resurrected and resumes leadership.