Objection 1. It seems that the Eucharist is not the principal of the sacraments. For the common good is of more account than the good of the individual (1 Ethic. ii). But Matrimony is ordained to the common good of the human race by means of generation: whereas the sacrament of the Eucharist is ordained to the private good of the recipient. Therefore it is not the greatest of the sacraments.
Objection 2. Further, those sacraments, seemingly, are greater, which are conferred by a greater minister. But the sacraments of Confirmation and order are conferred by a bishop only, who is a greater minister than a mere minister such as a priest, by whom the sacraments of the Eucharist is conferred. Therefore those sacraments are greater.
Objection 3. Further, those sacraments are greater that have the greater power. But some of the sacraments imprint a character, viz. Baptism, Confirmation and Order; whereas the Eucharist does not. Therefore those sacraments are greater.
Objection 4. Further, that seems to be greater, on which others depend without its depending on them. But the Eucharist depends on Baptism: since no one can receive the Eucharist except he has been baptized. Therefore Baptism is greater than the Eucharist.
On the contrary, Dionysius says (Eccl. Hier. iii) that "No one receives hierarchical perfection save by the most God-like Eucharist." Therefore this sacrament is greater than all the others and perfects them.
I answer that, Absolutely speaking, the sacrament of the Eucharist is the greatest of all the sacraments: and this may be shown in three ways. First of all because it contains Christ Himself substantially: whereas the other sacraments contain a certain instrumental power which is a share of Christ's power, as we have shown above (62, 4, ad 3, 5). Now that which is essentially such is always of more account than that which is such by participation.
Secondly, this is made clear by considering the relation of the sacraments to one another. For all the other sacraments seem to be ordained to this one as to their end. For it is manifest that the sacrament of order is ordained to the consecration of the Eucharist: and the sacrament of Baptism to the reception of the Eucharist: while a man is perfected by Confirmation, so as not to fear to abstain from this sacrament. By Penance and Extreme Unction man is prepared to receive the Body of Christ worthily. And Matrimony at least in its signification, touches this sacrament; in so far as it signifies the union of Christ with the Church, of which union the Eucharist is a figure: hence the Apostle says (Ephesians 5:32): "This is a great sacrament: but I speak in Christ and in the Church."
Thirdly, this is made clear by considering the rites of the sacraments. For nearly all the sacraments terminate in the Eucharist, as Dionysius says (Eccl. Hier. iii): thus those who have been ordained receive Holy Communion, as also do those who have been baptized, if they be adults.
The remaining sacraments may be compared to one another in several ways. For on the ground of necessity, Baptism is the greatest of the sacraments; while from the point of view of perfection, order comes first; while Confirmation holds a middle place. The sacraments of Penance and Extreme Unction are on a degree inferior to those mentioned above; because, as stated above (Article 2), they are ordained to the Christian life, not directly, but accidentally, as it were, that is to say, as remedies against supervening defects. And among these, Extreme Unction is compared to Penance, as Confirmation to Baptism; in such a way, that Penance is more necessary, whereas Extreme Unction is more perfect.
Reply to Objection 1. Matrimony is ordained to the common good as regards the body. But the common spiritual good of the whole Church is contained substantially in the sacrament itself of the Eucharist.
Reply to Objection 2. By order and Confirmation the faithful of Christ are deputed to certain special duties; and this can be done by the prince alone. Consequently the conferring of these sacraments belongs exclusively to a bishop, who is, as it were, a prince in the Church. But a man is not deputed to any duty by the sacrament of the Eucharist, rather is this sacrament the end of all duties, as stated above.
Reply to Objection 3. The sacramental character, as stated above (Question 63, Article 3), is a kind of participation in Christ's priesthood. Wherefore the sacrament that unites man to Christ Himself, is greater than a sacrament that imprints Christ's character.
Reply to Objection 4. This argument proceeds on the ground of necessity. For thus Baptism, being of the greatest necessity, is the greatest of the sacraments, just as order and Confirmation have a certain excellence considered in their administration; and Matrimony by reason of its signification. For there is no reason why a thing should not be greater from a certain point of view which is not greater absolutely speaking.
Source: Summa Theologica (New Advent)