That's a good question.
The Our Father is the prayer Jesus
taught us; but keep in mind that
He never said not to pray anything
else. In fact, He showed us a different
way of praying in John 17. He told
His disciples to go to Jerusalem
and pray for the Holy Spirit before
His Ascension, and at the Last Supper
He illustrated some of the prayers
we pray at the liturgy every Sunday.
Being a good Jew, He also must have
prayed the Psalms and other traditional
devotions. In addition, He sent forth
His Spirit upon His Church to inspire
them to pray in many different ways;
we see this in the saints throughout
So while the "Our Father" prayer
does have priority and is prayed
at every Mass, I don't think that
Jesus meant for this to be the only
prayer we pray.