Rubio would be terrible.
Rubio would be the most like Reagan in this race, IMO. Conservative but practical and will swing middle easily.
I see Cruz (my personal preference for over a year now) as more like Bush 2000... stronger conservative, with a little ron paul in there (caused an actual govt shutdown over the budget - he will stop the spending).
Trump is just so unpredictable. our allies will abandon us and lots of little things will change for teh worse. A president needs at least some level of serious gravitas. Trump is doing impressions of handicapped reporters, handing out Sen Graham's cell phone, calling other Repub candidates stupid, and this week, receiving criticism from leaders of UK, SA, Scotland, France... It's a potential train wreck. He's just not mature.