Aside from the Holy House at Loreto, the Portiuncula is my favorite too.
When I was little I used to make drawings of the chapel and then I'd construct models out of cardboard boxes. Outdoors I tried to make hermitages from branches and junk and pretend they were attached to the chapel. I'd just sit there or walk around, praying my rosary or a condensed version of the Little Office of Our Lady. As an adult, when I finally was able to visit the Portiuncula on pilgrimage, I was deeply moved.
A bit of history from Fr. Angelo Mary:
The beautiful Feast of Our Lady of the Angels, commemorates the dedication of the little chapel known as the Portiuncula (Little Portion) or Our Lady of the Angels. It was the third Church St. Francis rebuilt after Our Lord spoke to him from the Cross and ordered him to “rebuild the Church.” St. Bonaventure says that the three churches prefigured the three orders he later founded: First Order (the friars); Second Order (the Poor Clares); Third Order (the secular participants in the life of the Order). Thomas of Celano, the first biographer of St. Francis, quoted above, also writes:
For there had also been built in that place a church of the Virgin Mother who merited by Her singular humility to be, after Her son, the head of al the saints. In this church the Order of Friars Minor had its beginning, there, as on a firm foundation, when their number had grown, the noble fabric of the order arose. The holy man loved this place above all others; this place he commanded his brothers to venerate with a special reverence; this place he willed to be preserved as a model of humility and highest poverty for their order, reserving the ownership of it to others, and keeping only the use of it for himself and his brothers. - Mary Victrix
As is well known, St. Francis obtained from Our Lady what is known as the Portiuncula Indulgence - or Great Pardon of Assisi - which can be gained by any of the faithful on The 2nd of August from Midnight to Midnight. For information on the plenary indulgence, go here.