I'd go with Michael. I respect Vito but I think Michael had just as much intelligence and gift for strategy, but with the extra skill of knowing when to kill. Michael would put you in a body bag quick and his calculated use of murder saved the family more than once.
Vito made a huge mistake, imo, sending Luca Brasi to spy on Sollozzo which caused a cascade of issues for the family, so that's a big strike against Vito. I don't think Michael made any decisions that bad. What missteps did Michael really make?