My default is G1 Fort Max. However, my favorite changes every day, depending on what I feel like tinkering with. I think TR Fort Max could take the spot with enough 3P upgrades (reprolabels, guns, sword, and an update for Cog)...
As a kid it was G1 Metroplex. Now it is probably Takara MP Grimlock. I love that apron and drinks tray.
Good choice. Great figure. My favorite is the original Ultra class Optimus Primal. The new MP coming out soon might 1 up it.