is the very first commandment the most important of all?

Shalom, Thank you for your question. The quick and simple answer to your question is – no. Now, let’s explain a little more. Firstly, it isn’t so simple to figure out what is the 1st commandment. The first one that appears in the Torah (having children). Or perhaps the first one given to the Jewish people as a people (making a calendar) [see the first Rashi on the Torah who suggests that the Torah perhaps should have started with this command]. Or the 1st of the ten commandments. Or the 1st mitzvah on the list of commandment of the Rambam. As you can see we have a lot of choice here. Next, even if you pick one of these “firsts” we would still no be able to say that it is the most important. What does “important” mean? In Pirkay Avot (Chpater 2,1) it states “be [as] careful with a “light” Mitzvah as with a weighty one, for you don’t know the reward given for [each of the] Mitzvoth.” So, if we are talking about importance in relation to reward – the Mishna tells us that we really do not know. Do we mean importance from the point of view of spiritual growth, or the needs of the Nation of Israel, or for this time and place, or for this person? Here too we have many options. In general, we understand that the entire 613 mitzvot, and the complete Torah are one unit that each part of gives value to the whole. I hope this is of some help, Blessings.