It's interesting to note that more than a year ago the Clinton camp feared Little Marco and Jeb! the most, and (disclosed in an email, of course) decided to use their considerable media clout and contacts to really push and give a serious voice to the three radicals, Carson, Cruz, and Trump. We won't know for years, or perhaps ever, what effect they may have achieved, if any, but of course we do know that Carson led briefly and Trump and Cruz finished 1-2.
I'd say without Trump in the race at all, it was likely to be Marco or Jeb! and while both had flaws (Marco was exposed as very inexperienced and Jeb! isn't very good on stage, plus a Bush) I'm not sure those rise to the level of HRC's baggage and ongoing scandals which have been muted by Trump's major gaffes.
I'd speculate Jebbers could have maybe pulled it off. He's moderate, (more of a '92 Bill) strong on policy, has a clean record, has broad Latino appeal, (he'd win Florida by 10 points) and his last name would help hold the hardcore R's and Evangelicals together in a general election. And against Hilary his weakness of being part of a "Bush Dynasty" would be reduced by comparison. He'd have outdrawn independents when push came to shove, I think.
A big problem for generic R's and regular right-leaners is that they've let the far right control the primary by sitting on the couch while the Evangelicals and Tea Party turns out en masse and pushes the nominee into crazyland in recent cycles. Maybe that will change, but I can't say I'm confident there's not more apathy on the horizon.