I don't think there was any other Republican in the field that could have beaten Hillary Clinton. This isn't a reflection on the capabilities of the candidates, but on the peculiarities of this campaign and on Trump's path to 270 electoral votes.
Pick a generic Republican candidate this year (Scott Walker, Rick Perry, Marco Rubio, or Ted Cruz), and ask yourself two questions:
1. Would this candidate have won any states that Trump lost?
2. Would this candidate have lost any states that Trump won?
I sat down and went through this a few times, and I had a hard time convincing myself that the number of electoral votes the generic GOP candidate might have gained in #1 was enough to offset the likely loss of EVs in #2.
For example: I don't think there was a single candidate other than Trump who would have won Pennsylvania.
Interestingly, I think any of these generic GOP candidates would have outperformed Trump in the popular vote, only to lost anyway -- because they simply would have gotten bigger GOP margins in deep "red" states.
>>For example: I don’t think there was a single candidate other than Trump who would have won Pennsylvania.<<
Well, for starters, Senator Toomey distanced himself from Trump to an extent and still got more votes than Hillary did, although a few thousand less than Trump. Granted, he was an incumbent and wasn’t a presidential candidate, but here’s what I find interesting.
Toomey’s lack of outright support for Trump cost him in coal country where Trump outperformed him, but Toomey managed to get more of the suburban GOP vote than Trump. I find this encouraging.
View this election as a river of potential GOP voters. Upstream there are four tributaries, with by far the largest being composed of the traditional GOP voter who supports the eventual nominee, regardless. A much smaller tributary feeding the GOP vote is composed of disaffected Democrats, e.g., coal union members, etc., who Trump, like Reagan, pulled into the GOP stream, diverting it from the traditional Democrat stream, a two-fer. Depending upon a lot of factors, this stream could dry up, or increase, in 2020 and beyond.
The other two tributaries hold GOP voters, but of drastically different temperaments. The Toomey stream is composed of voters who couldn’t abide Trump’s perceived character flaws, and because of that just couldn’t defend a vote for him, even though they easily voted for Toomey.
The Trump stream is composed of anti-establishment (anti-Bush?) GOP voters who would have stayed home or written in a third party if Bush, or Kasich, or ???, were the nominee. Trump won partly because this stream has grown over the past six years of dissatisfaction with the GOP Congress, and because he managed to divert a significant portion of the the DEM stream, with Hillary’s help.
Here’s where I see it going, assuming Trump does just a reasonable job as President the next four years: The Toomey stream and the Trump streams will merge into the rest of the massive GOP stream, and the diversions from the DEM stream will continue. Third party voting by former GOP voters will be way down, and the GOP will be in excellent shape in 2020.