My bet is Sununu would comfortably win against both Desantis and Trump, and narrowly against just Trump.

If Sununu doesn't run I think Trump would beat Desantis narrowly, but I wouldn't be surprised if Desantis won, especially if he gains more popularity with Trump's base over the next nine months. If both Sununu and Desantis don't run Trump will probably win NH easily.

Other than Trump dropping out, I can't imagine a scenario where anyone outside of those three would win the primary.