The most important of the seven sacraments and central ritual of the mass is

The seven sacraments are
Baptism, Confirmation, Eucharist, Penance, Anointing of the Sick, Holy Orders, and Matrimony.
The most important of the seven sacraments and central ritual of the mass is the Eucharists.

This is because Christ personally commanded all Christians to celebrate the Eucharists.
The Eucharists is Christian service, ceremony, or sacrament commemorating the Last Supper, in which bread and wine are consecrated and consume.