Frankly, I don't think Cruz would have done as well as he did had Trump not been in the race. Once Trump broke the mold about how primaries are supposed to go, I think more voters decided to look outside the box and reject the chosen candidate (Jeb). Also, I think Trump emboldened some of the other republicans to atack the moderators at debates, etc. Had Trump not been in the race, I think it would have been 2012 all over again.
Carson too. He wouldn’t have lasted a week without the icebreaker Trump.
There’s no opponent Cruz could have beaten one on one - he’d have looked and sounded the prick standing next to every one of them - even Rand Paul. There’s no opponent Kasich could have handled one on one in republican primaries - he’s a Democrat that believes in balanced budgets.