"In Baptism and the other sacraments, except the Eucharist, we are united to Christ simply by a participation of His grace, but in the Eucharist, Christ exists substantially, and is really and truly present--Body, Blood, Soul and Divinity. In the Eucharist, man realizes more fully his incorporation to the Death and Resurrection of Christ than [...]
"By his command to 'do this in remembrance of me', he asks us to respond to his gift and to make it sacramentally present." Source: Pope Benedict XVI, SACRAMENTUM CARITATIS (Sacrament of Charity), 2/22/2007
"As to the effect, considered in each of the partakers. For, as Ambrose (Mag. Sent. iv, D, xi) says on 1 Cor. 11:20, this sacrament 'avails for the defense of soul and body'; and therefore 'Christ's body is offered' under the species of bread 'for the health of the body, and the blood' under [...]
"The inexhaustible richness of this sacrament is expressed in the different names we give it. Each name evokes certain aspects of it. It is called: The Holy and Divine Liturgy, because the Church's whole liturgy finds its center and most intense expression in the celebration of this sacrament; in the same sense we also call [...]
"None [of the other sacraments] can be compared to the Eucharist… all this [Baptism, Reconciliation, Confirmation, Anointing of the Sick, Holy Orders and Marriage] seems to be only an apprenticeship to His love for men. In the adorable Sacrament of the Eucharist He goes much further." Source: St. John Vianney, The Meditations of the [...]
"A sacrament is so termed because it contains something sacred. Now a thing can be styled sacred from two causes; either absolutely, or in relation to something else. The difference between the Eucharist and other sacraments having sensible matter is that whereas the Eucharist contains something which is sacred absolutely, namely, Christ's own body; the [...]
The Eucharist is "the source and summit of the Christian life.” "The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate, are bound up with the Eucharist and are oriented toward it. For in the blessed Eucharist is contained the whole spiritual good of the Church, namely Christ himself, our Pasch." [...]
Absolutely speaking, the sacrament of the Eucharist is the greatest of all the sacraments... because it contains Christ Himself substantially. Source: St. Thomas Aquinas, Summa Theologica