The answer is Cruz, not Rubio.
Here are some interesting polling results reported by Ezra Klein at vox.com. (h/t Daily Kos)
"If Trump and Carson falter, Cruz benefits"
Cruz beats Rubio 27% to 15% when asked about a second choice. The rest of the pack is far behind.
This question tests something interesting: Do GOP voters like Trump and Carson because they're uniquely compelling candidates? If so, they may well see Rubio as the next most compelling candidate, even though he is pretty stylistically and substantively different. But another possibility is that Republicans like Trump and Carson because they're combative, outsider conservatives with hard-line positions on key issues, in which case Rubio — a genial establishment favorite who has a real record of working with Democrats — probably isn't a fit.
The results here suggest the latter. Of the candidates in the race, Cruz is closest to Trump and Carson in ideology and approach, and it looks like he is the main beneficiary if they drop out of the race.
"If the field narrowed to just Donald Trump and Marco Rubio, Trump would crush Rubio 57-43."
... in a head-to-head matchup among Republican voters, Trump beats Rubio 57-43. That suggests that Trump's ceiling, at least among Republicans, is far above his current 25 to 30 percent, and he may well benefit as weaker candidates drop out.
"Ben Carson and Ted Cruz lead the second-choice sweepstakes"
One possible argument for Rubio is that though he may not be many voters' first choice, he could prove to be a lot of people's second choice, and so his strength will reveal itself as other candidates prove unacceptable or drop out.
At the moment, though, there's relatively little evidence of that: Rubio and Trump are within a percentage point of each other [8% vs. 7%] when it comes to voters' second choice, and Ted Cruz and Ben Carson are ahead of both of them [11% vs. 12%].
None of this means Rubio is cooked. It's only to say that for all that the conventional wisdom and the betting markets favor Rubio, it is really hard to find support for his strength in the numbers.
The bad news, if Trump and Carson drop, is that the rest of the pack are cut from the same mold.