Have a few candidates.
Rodimus has always been my favourite Prime. Thought Furman done a good job with him in the Marvel comics. I'd love some fiction that picks up from the Marvel UK future stories. Having grown up with Marvel UK and the Movie, Rodimus will always be Optimus' successor to me.
I've always liked Prowl. I like Roche's devious version but I liked the kinder version in the Budianksy and Dreamwave comics. I just wish writers would stop making him a whiner. I liked his tech specs portrayal of being patient and a listener, not a talker. At least with Energon Checkpoint, we have an idea of what he'd look like as a Prime. If you go by Beast Wars, he becomes joint Maximal leader.
Marvel Blaster. The Autobots wanted him to lead.
Silverbolt. Always liked him and like Prowl, he also becomes joint Maximal leader.