It was the Reichstag fire that made it possible for Hitler to pass the Enabling Act. By blaming the fire on Marinus Van der Lubbe, a mentally unbalanced arsonist, and four members of the Communist Party, he was able to play on the fears of the German people that communists were attempting to overthrow the German republic.
It was in this context, in the wake of the fires, and amid a storm of propaganda by the Nazi Party, that Hitler was able to pressure President Paul von Hindenberg to sign off on the Enabling Act, legislation which gave virtual dictatorial powers to Hitler. He used these powers to outlaw the Communist Party and imprison members of other opposition parties, as well as seizing control of various media outlets. The legislation was supposed to be temporary, but was made permanent by Hitler. When Hindenberg passed away, he consolidated the offices of President and Chancellor, and became the undisputed leader of Germany, now a single-party dictatorship. We cannot, of course, know if the Enabling Act could have been passed without the Reichstag Fire, but it seems clear that Hitler's skillful manipulation of the fallout from the fire facilitated his rise to power.
One slight revision: Hindenberg (reluctantly) named Hitler Chancellor on January 30, 1933, about a month prior to the Reichstag fire. The Enabling Act invested him with the powers mentioned in both previous answers, but it did not make him Chancellor.
It was the Enabling Act which named Hitler as Chancellor; the Reichstag Fire allowed further curtailment of civil liberties which helped the National Socialist Party gain a controlling majority in the Reichstag. For that reason, it is difficult to ascertain if one was more important than the other--both were signficant developments.
Article 48 of the Weimar Constitution provided:
In the event of a state not fulfilling the duties imposed on it by the Reich, the Constitution, or the laws of the Reich, the President of the Reich may make use of the Armed Forces to compel it to do so.
If public security and order are seriously disturbed or endangered within the Reich, the President of the Reich may take measures for their restoration, intervening if need be with the assistance of the armed forces. For this purpose, he may suspend for a while, in whole or in part the fundamental rights provided [in other provisions of the Constitution.]
President von Hindenburg, relying on this provision, named Hitler as Chancellor under the Enabling Act with dictatorial powers to deal with the economic crisis Germany was then facing. The Reichstag Fire happened six days before elections which Hitler had called because he did not have a majority in the Reichstag and had been forced to form a coalition government. The fire allowed the National Socialists to persuade President von Hindenburg to sign the "Decree for the Protection of People and State" which evoked the powers of Article 48 further. The Nazis used this to intimidate opponents in the election and thereby secured a majority in the Reichstag. Hitler claimed the fire was the first step in an attempted Communist coup. There was speculation for many years that the Nazis themselves set the fire as a pretext to gaining power; it has since been determined that the fire was indeed set by a Communist sympathizer. It could not have played out better for Hitler and his cronies.