Putting aside all partisan differences (as if we could), let's ask what a man or woman needs to do to become a successful President.
It's not as simple as drawing big crowds more than a year before the election, or effectively starting to run years before, or even carefully avoiding holding public office in order to appear pure to the purists.
The successful President has to be able to do three things. Each of these things requires different talents and skills.
First, the would-be candidate must win the party primary. This means appealing to the base, the party faithful, who nowadays inhabit the extremes of the political spectrum, left and right. Between both parties, they add up to about 20% of the electorate (that is, of all eligible voters). BUT, most of the time, the parties sober up and nominate an electable candidate.
Next, the nominee must win the general election. Now, the two bases amount to a smaller percentage of the electorate and so hold less sway. The parties' main job is to discourage voters likely to support their opponent from voting, and this is the purpose of negative TV advertising.
Third, the President must be able to govern effectively. This is impossible without good judgment, a capable staff and the ability, somehow, to work with a divided Congress (or a divided party, in the case of the Republicans).
Most people shortsightedly look at these three phases as if they functioned separately. So, just for fun, let's see who stands out at each phase.
Appealing to the base. That would be Donald Trump, of course, although I'm sure that when his star starts to blaze out he will pick up his marbles and go home. Cruz and Jindal seem to be pinning their hopes on the religious right, as is Carson. None will win the nomination. Bernie Sanders is also appealing to the left wing of the Democratic Party. I don't think he can sustain the pace to be nominated.
Win the general election. The way things are going, Joe Biden would have an easy time of it. Of the Republicans, Bush might do it, except that he has been gaining the reputation of a sincere doofus. Besides, his party's right flank will do everything it can do to stop him. Mario Rubio maybe just could do it. Carly Fiorina, I think not.
Definitely, in spite of all the bad press, Hillary Clinton could win. Plus, she would govern well. . . as a technocrat, for that is what she really is.
All those listed above as capable of winning the election could govern, I would say. None would make a great President. What is a great President? One who has worthy, even lofty, goals; is able to achieve them; and leaves a legacy behind him. I can think of only two: Lincoln and FDR. Reagan comes close.