Not many people are answering the actual question.
The president nominates and the senate advises and consents. This spreads out the power from any one person or one group. It is part of the checks and balances built into the constitution to stop many of the problems that the founders saw in other governments.
Only congress can vote for laws. The president can't, the supreme court can't. Thus, when it comes to laws, congress gets to control it (only bills approved by congress will ever become laws). But, congress can't just force laws through; the president must also approve (terrible congressional ideas can be vetoed). But if you have a crappy president, an overwhelming majority of congress can override that crappy president. Finally, if a truly bad idea gets through both, the supreme court can override it all. In this situation, the congress has the power, but the other branches can override them.
Only the president can nominate a supreme court justice. It is the reverse of the case above--the president gets to control it (only candidates that the president wants will ever become supreme court justices). But if the president chooses a really bad candidate, the senate can advise against that candidate. And if the senate and president do so illegally, the supreme court can override them. In this situation, the president has the power, but the other branches can override them.
The supreme court has the final say in all things, not the congress and not the president. But to prevent too much power gathering in the supreme court, they can't nominate themselves. The power to do that comes from the other branches. In this situation, the supreme court has the power, but the other branches can eventually replace them.
In all of the cases that I listed, a different one of the three branches has the power, but never absolute power.