Disclaimer: I was a Bernie voter in the primaries, and voted for Hillary in the general election. I'm also of a mind that anyone would be better than Trump. Anyone at all.
Having said that...
Ben Carson? Just hearing his name kinda puts me to sleep. He wasn't a good candidate. He mumbled. He sort of looked like he didn't know what was going on most of the time. I don't know that he ever really got any good ideas out there. I don't think he would be a good choice.
How 'bout Marco Rubio? He's young, and would be a great end to the Republican legacy of putting forth old, rich, white dudes as candidates. I live in Texas, where the Democrats are relying on the Hispanic vote to turn our state from red to blue. Having a Hispanic Republican President might put an end to the idea that the Democrats have a lock on minority voters, just at a time when, in some places, people of color are becoming the majority.
I don't know if Marco could have beaten Hillary, but I'm pretty sure he would have given her a run for her money, in the same way Obama did in 2008, appealing to younger voters.