The Supreme Court is effectively the last word, accountable to no-one. Five justices form a majority on that court, giving them the closest thing we have in this country to dictatorial power. Indeed, at times they do seem to have dictatorial powers. Their decisions stand, and the voters have no means of changing those decisions.
Voters must consider carefully what kind of justices the next president is likely to appoint. That president will probably appoint at least three new justices. Obama's two appointees, Sotomayor and Kagen, will likely remain on that court for decades, as will those three new justices. If the new appointees are similar to the Obama appointees, that will give them decades of absolute majorities on that court, the ability to impose unchangeable rulings on the people.
So which candidate is most likely to appoint good justices, rather than leftist sycophants? Certainly not either of the democratic candidates. Clinton or Sanders would appoint leftists to that court. Trump? That is essentially unknown, the only hints we have are his history of supporting things like condemnation of private property to give it to other private entities, and federally controlled health care. The probabilities do not look good, it is doubtful that he would appoint defenders of constitutional freedom to any court.
Of course there are always a lot of unknowns in electing a president. However, I believe that the candidate most likely to appoint good justices is Ted Cruz. Of all the candidates, he is most committed to the constitution. Voters must consider that when deciding how to cast their ballots.