The head of all the Mitzvos:[1]

Chazal state that the Mitzvah of Tefillin is connected to the entire Torah. The purpose of all Torah and Mitzvahs is to draw G-d’s unity down into the worlds. Although this is accomplished through fulfilling all Mitzvos, nevertheless by the Mitzvah of Tefillin has a greater it is more revealed, as the Parshiyos inside the Tefillin discuss G-d’s unity. Hence it is the head of all the Mitzvos.

[1] Likkutei Torah Emor 1^st Mamar p. 33