"But, but, amnesty, amnesty..." Immigration is a problem, but it is not our biggest problem. It is amplified by the poor economic growth of the past few years, which in turn were a direct result of Obama's economic policies. Fix the economy, and we will have more jobs than workers again. We will need immigrants.
We do need to fix our immigration system, and decide who gets to come here, and who has to leave. In that, Trump actually supports "amnesty", as he has said exactly what his supporters attack other conservatives for saying: illegals need to return to their home countries and apply to return here.
Those who support the #NeverTrump campaign do so because they believe that Trump is not a conservative, is temperamentally unsuited to the presidency, has no actual solutions beyond superficial sound bites, is incapable of winning a general election, or some combination thereof. None of those concerns exist to the same extent with any other candidates - and yes, I would support any other Republican over Donald Trump.
Immigration is the biggest problem.