Most Christians do not see it this way: Easter and Pentecost ranking above Christmas. It’s not that there is some kind of competition among observances. But feasts that draw out some mystery or aspect of Christ are worth the special attention of believers.
77. A similar commemorative logic guided the arrangement of the entire Liturgical Year. As the Second Vatican Council recalls, the Church wished to extend throughout the year “the entire mystery of Christ, from the Incarnation and Nativity to the Ascension, to the day of Pentecost and to the waiting in blessed hope for the return of the Lord. Remembering in this way the mysteries of redemption, the Church opens to the faithful the treasury of the Lord’s power and merits, making them present in some sense to all times, so that the faithful may approach them and be filled by them with the grace of salvation”. (Sacrosanctum Concilium 102)
After Easter and Pentecost, the most solemn celebration is undoubtedly the Nativity of the Lord, when Christians ponder the mystery of the Incarnation and contemplate the Word of God who deigns to assume our humanity in order to give us a share in his divinity.
And some Easter Christians would rank Epiphany, the revelation of the Lord to the world, as equal in importance.