Two thoughts:
The answer to a candidate who COULD have won against Trump is clearly Hillary. All she had to do was take the campaign seriously, not try to run up the score, and just spend all her time talking to voters in the rust belt. Also, never get caught on a mic describing potential Trump voters as “deplorables”. She ran a (in retrospect) horrible campaign, and she still easily won the popular vote and came within a football stadium of winning overall. She obviously “could” have won.
The answer to the question of who WOULD have won is, most likely, any other serious candidate, particularly (to be cynical) a safe-seeming white man. Who would that have been? Beats me. But if Hillary has a heart attack and dies in 2013, and Biden is still sidelined by family issues, then SOMEONE is going to run on 2016, and whoever it is, I think they could easily have beaten Trump. All the people who realized how awful Trump was would have voted for him. All the normal democrats would have voted for him. And all the people who had been affected one way or the other by decades of anti-Hillary hate who stayed home rather than voting for someone they hated so much would have voted for him.
The big question mark is, of course, Bernie. Now, I’ve given up trying to really prognosticate… maybe he would have caused absolutely massive, historic turnout among the young. But I think the “he’s been in the public eye for so long, all the negatives are already known” argument is specious in the extreme. Note that despite Trump’s love of giving his opponents denigrating nicknames (Pocahantas, Sleepy Joe, Ron DeSanctimonious, etc) he never really laid into Bernie. Why? Because he couldn’t think of one?
Remember when Bernie was actually the leading democratic candidate early in the 2020 primary season? Why wasn’t Trump (and Foxnews) unloading on him with both barrels of the sewage cannon? I mean, Trump was so scared of Biden that he literally committed treason and got himself impeached trying to get Ukraine to manufacture dirt on Biden. Why wasn’t he even more scared of Bernie if (as some seem to think) Bernie was an even better candidate against Trump than Biden?
The obvious answer to that question is… because they were just praying that Bernie would become the nominee and they could just unload on him.
Now, would he have won anyhow? I mean, who knows? Maybe he would have actually gotten a massive groundswell of young voters. Maybe all the anti-establishment people who had voted for Trump in 2016 would have come out for Bernie and just turned all conventional predictions on their ear. But… I honestly believe that any such electoral plusses Bernie could have brought would have been swamped by the tidal wave of middle America to which communism and the Soviet Union is still the capital-E-Enemy of America. Do they tolerate, and maybe even enjoy, crazy-Uncle-Bernie who says things they mostly agree with and reminds them of Larry David? Sure. Are they aware that he used to have a hammer-and-sickle flag in his office, and took his honeymoon in the Soviet Union? Well, if they aren’t now, they sure would be after several months of those being literally the only talking points on Fox News.