Has to be Masterpiece Optimus. Of all the Optimus Primes, when he came out, he had the biggest impact. He actually altered how people drew G1 Optimus and, generally, has BECOME the standard for what G1 Optimus looks like. He has the Diecast and rubber tires of G1, the poseability of Classics, the features of a highly detailed, non-transformable model kit of statue, and has engineering that stands up to the best toys of today. That's not to say I don't love the others. Of any major lines, Classics Optimus is my favorite, for being the most fun, best gimmicks, and just generally looking good with other toys of his line (Rather than dwarfing them mercilessly). But when the name 'Optimus Prime' is mentioned, it's still MP Optimus that pops into my head first.