Wow, this question is somewhat difficult to answer.
The Gospel according to St. Luke would be my choice because the first chapters reveal the Joyous Mysteries of the Holy Rosary, among other many reasons. But then again, I would ultimately change my mind and choose the Gospel according to St. John, which is so sublime in its ethereally theological presentation of such sacred Mysteries as that of the hypostatic union of Christ's human nature with the Eternal Word, the only-begotten of the Father (the Last Gospel we all hear at the end of Mass), the Holy Eucharist, indwelling of the Blessed Trinity in the souls of the just, &c.
My most favorite Gospel lesson would be the Parable of the Prodigal Son (St. Luke, ch. xv., 11-32), which is so consoling to read before Confession. It is the Gospel lesson for the Mass of Saturday in the Second Week in Quadragesima (this coming Saturday).
But then again, a favorite Gospel lesson is that of Our Lord's discourse to the Samaritan woman (St. John, ch. iv., 5-42), which is found in the Mass for the Friday in the Third Week in Quadragesima.
But then again...
I have to stop now, or else I will never stop...