When we list the Matriarchs in prayers (such as the Amidah in the version including the matriarchs, or in the traditional blessing for one's daughters on Friday night), we say them in the order: Sarah, Rebekah, Rachel, Leah. Why is this? The patriarchs are generally listed in order of age, which seems like a natural order of precedence; and similarly, apart from Leah the matriarchs are listed in order of age.
Likutei Dinim Toras haKohanim Siman 75 says that since Rachel was the one Yaakov had in mind to marry first therefore Rachel is listed first.
Rachel was certainly Yaakov's primary wife; the text indicates that his primary residence was with her and only changed once she passed away. She was the one he desired and labored for and set his focus upon. Although Leah bore him more children than any of the other women, it's clear that he loved Rachel more.
From a more kabbalistic perspective, Rachel represents the natural, revealed world, whereas Leah represents the hidden, deeper dimension of reality. We give precedence to the revealed before the hidden, generally speaking.