I'm not sure I'd prefer any of them to be prez, but I think the ideal for debates would be Hillary vs Bush with Trump as independent. Trump would destroy Hillary (or at least get a really great sound byte / reaction) and the Bush/Trump exchanges would be mildly entertaining as well.
Would practically guarantee a Clinton presidency tho.
If we're talking pure comedy gold I think you want a Bernie, Trump, Jeb stage. I am imagining Bernie calling Jeb low energy, Bush calling Bernie a white haired ninny muggins, and Trump just watching making one of his six default facial expressions.