Optimus Prime. Mainly because I loved the Beast Wars / Beast Machines saga, in which his descendent, Optimus Primal featured (as a - possibly gray-backed - gorilla). Plus, I remember watching one of the first Transformers cartoon series way back when (couldn't have been 1987, since I was born that year).
Also, my Transformers' stuff easily outnumbers my SM stuff (find the double-entendre in this sentence, if you dare ). Though my favourite Beastie was Cheetor, not Optimus, my vote still goes to him. (Plus, my favourite SMB character is Yoshi.)