I disagree here once you get beyond the top 2. HRC and The Donald will most likely be the nominees and in that event, HRC will most likely be the next POTUS.
But until the race is actually over, I don't see how you can put Ryan above Bernie and Kasich. Ryan isn't running. So unless a brokered convention actually happens and the Repubs select him, he has 0% chance at this point in time of being the next POTUS. For me personally, I'd flip Bernie and Cruz. Both are trailing HRC and Trump respectively for their party nomination. But in the event Bernie catches/passes HRC, his electability ratings trump (pun intended) all other candidates, meaning he has a better shot at winning the General Election.
IMO it's:
HRC
Drumpf
Bernie
Cruz
Kasich
Ryan