In no particular order. If you held a gun to my head and had me pick one, Trajan.
I think Trajan is probably the most notable of the Roman Emperors. He is the second of the 'Three Good Emperors'. He both expanded the Roman empire and understood that his own genetics played very little role in how he accomplished such a feat on his own part. He knew, somehow, that his memes were superior to his genes. He demonstrated this knowledge when he picked Aurelius as his successor.
Sadly, Aurelius, for as wise as he was, was unable to see the error that he made when he appointed his own son Commodus to the throne.