Probably needs to win first: Donald Trump. If Trump's polling lead in New Hampshire melts to a second-place finish, Trump risks looking like a paper tiger. Not only will the Iowa debacle be repeated, but he will have given up an even larger lead than he had in Iowa. If Trump can't win in New Hampshire, a state with a large independent and maverick-y primary electorate, his path to the nomination narrows considerably. Trump has shifted his strategy a bit since Iowa, but we'll see tomorrow if he shifted it enough to win.
Depends on a strong second-place finish: Marco Rubio. Even though he had been in third place in the Iowa polls for weeks, Marco Rubio's campaign was able to use a stronger-than-expected showing in Iowa to grab the Big Mo. After Iowa, Rubio's polling numbers rocketed up in the Granite State, and, by the middle of last week, it looked like he might be on a trajectory to catch Trump. That ascent has been interrupted, but Rubio's campaign has projected a second-place finish in New Hampshire for a while. If Rubio finishes within 5 or so points of Trump (assuming Trump's first) and outpaces his rivals by a similar margin, he'll be better able to make the claim that he is the center-right candidate who unifies the party. A strong second place by Rubio would put a lot of pressure on Bush, Christie, and Kasich to start inching toward the exits. A weaker finish, though, would be an expectations loss for his candidacy.
Needs to be close to or better than Rubio: Jeb Bush, Ben Carson, Chris Christie, Carly Fiorina, and John Kasich.
There are variations in the stakes of a strong performance in New Hampshire for each of the three governors. In order to have a path forward, Christie and Kasich both probably need either to do better than Rubio in New Hampshire or to come within a few points of him. Both have centered their campaigns around New Hampshire; according to one count, Christie has spent more days campaigning in New Hampshire than Carson, Rubio, and Trump combined. Without getting close to or beating Rubio on Tuesday, they'll have a hard time marshaling the resources to compete in later states. (There is a slight variant here: either Christie or Kasich could also help maintain their viability even if they come substantially behind Rubio as long as they also beat the candidates below them by a solid margin.)
Jeb Bush is in a slightly different situation. He has a substantial campaign infrastructure and considerable financial resources. Bush's team seems to be digging in to compete in later states no matter what, so New Hampshire might not be a do-or-die state for him. However, a poor performance in New Hampshire would cause much of the political establishment to unleash intense pressure on Bush to drop out. On the other hand, Bush has essentially been left for dead by much of the chattering class, so even a double-digit result could be seen as a sign of political revival. If Bush gets close to Rubio, his campaign could argue that Bush has regained momentum. Rubio's allies are obviously concerned about that possibility: Rubio's Super PAC has been spending big against Bush recently.
Fiorina and Carson already face significant obstacles in the GOP primary. They can hang in the race for a while, but double-digit finishes in New Hampshire would help reinvigorate their campaigns.