We can't really include YET, because we've only seen him as a Don for like 15 min. And I don't think he was the wrong choice, but the only one.

Vito was legendary, I can't say enough on how he built his empire into the biggest family in the U.S. (Or at least in New York...)

Michael was meticulous, very decisive with every move, and ultimatley led the family into it's biggest success.

So I say Mike.