What activity is least important to the good practices of Judaism?

Answer 1

This question fails to understand what Judaism is about. There are not some duties that are of a higher calling than other duties (as opposed to Catholicism's Sacraments which are holier than other requirements). Judaism is a combination of practices which in concert facilitate the elevation of the soul. Therefore each is as necessary as every other. Shomer Shabbat, Kashrut, Tzedaka, Gemilut Hasidim, Purity, and numerous others are equally and strongly important. The best non-religious example I could give is to imagine a rope with three braided strands. Each one of the three is just as important as the other two for maintaining strength.

Answer 2

Judaism attaches various levels of importance to its observances. Least important are things that ostensibly have nothing directly to do with one's Judaism, such as choosing which of two bus routes to use.

More important than that are minor customs and mannerisms such as using Jewish forms of speech. More important than that are customs that are listed in the Jewish code of law such as eating hot meals on the day of Shabbat.

More important than that are the Rabbinical decrees and commands such as lighting the Hanukkah candles.

More important than that are the commands within the Torah itself, such as not eating leavened foods on Passover.

Most important are: belief in God, not believing in idolatry, and avoiding adultery and murder.