If you want to really know who is feared in a campaign, follow the money. The money shows right now that for all the media attention around Ted Cruz and Donald Trump and all the poll analysis, the candidate who is being targeted the most by the others with attack ads is Marco Rubio, while Cruz is facing very little in the way of attack ads and Trump is facing zero:
What is unusual is that the Florida senator is absorbing the lion's share of attack ads, across multiple states, being aired by super PACs supporting other Republican candidates despite running third in most public opinion polls. The front-runner, Donald Trump, has confronted none. Literally, zero dollars in super PAC advertising have been run against him this month, according to Federal Election Commission filings.
...For that matter, only a minimal amount of critical ads were directed at Cruz, who is running second in most polls and tied for first with Trump in Iowa. Pursuing America's Greatness, a super PAC supporting long shot candidate Mike Huckabee, who won the Iowa caucuses in 2008, is the only one to target Cruz.
Now, there are reasons why this is happening - other candidates with money (especially Jeb Bush) want to run Rubio out of the race because they are playing a game of chicken in which they wish to leave the voters no other alternatives to Trump or at least to Trump or Cruz. Everybody still in the race seems to think they can take Trump in a one-on-one race, whereas absolutely everybody's last choice of who they want to face one-on-one is Rubio, since he's the one conservative candidate who can appeal to establishment and moderate voters and the one "establishment lane" candidate who can appeal to conservative voters.
The failure of anyone to attack Trump over the airwaves is irrational and not in the GOP's best interests as a party, since he would clearly be a catastrophically bad general election candidate, and while I don't believe he'll be the nominee, you can't make that happen without trying. We have vast amounts of evidence from past campaign seasons that negative ads work, and no rational basis for believing that they would have no effect on at least dampening Trump's ability to add new supporters. The one sustained campaign run against Trump earlier in this campaign, by the Club for Growth in Iowa, coincided with the first time Trump lost a lead in the polls anywhere. There may be debate over what the best angle of attack would be, mostly because Trump presents such a wealth of targets, but that's why you test out a few options and run with what makes the biggest dent. This is not rocket science, it's how campaigns have been run since time immemorial, in general elections and in both sides' primaries.
I realize we're at the peak of the season for everybody and his uncle trying to claim the mantle of the guy "the establishment" hates - as if the GOP establishment was a monolith rather than a loose confederation that couldn't organize a potluck supper, much less a political campaign (lest we forget, we got to this point in large part because people lost faith that the party establishment was even competent at doing the things we expect party establishments to do). But if you want to know which candidate everybody else in the race truly fears, which candidate people are willing to invest donor money in attacking, follow the money. It's not Trump. It's not Cruz. It's Rubio.