Hands down it is the Easter Vigil. Catholic Culture has a short definition of what this liturgy includes:
The ceremonies of Holy Saturday and the most solemn memorial of the liturgical year. They consist of four parts: Service of the Light, Liturgy of the Word, Liturgy of Baptism, and Liturgy of the Eucharist. The entire celebration takes place at night, and therefore it should not begin before nightfall and should end before dawn on Easter Sunday. In the early Church the night before Easter was celebrated by the Illumination of the churches and even of whole cities. The revised Easter Vigil services include ceremonies that go back to the first centuries of the Christian era and stress the Church's joy in commemorating the night that Christ rose from the dead.
Growing up, despite the fact that I attended Mass weekly with my parents, went to Catholic school beginning in sixth grade, and was a church musician starting from the age of 15, I don't think I even knew the Easter Vigil existed! My family generally attended the Saturday night Mass every week of the year except Easter.
Then I went off to graduate school and, as a way to get involved in something not related to my major in any way (and thus keep what little sanity I had), I got involved with the RCIA team. We had some people joining the church "from scratch"--they hadn't ever been baptized. Others were already baptized Christians who were converting, or maybe even baptized Catholics who hadn't completed their sacraments. The culmination of their year or more of preparation is the Easter Vigil, so I was required to attend.
If you're going to your very first Easter Vigil, the Basilica of the Sacred Heart is the place to go (click on the Virtual Tour to see why!) This is not a liturgy for wimps. This celebration calls for all the grandeur the Church can bring to it. This is THE celebration of the year!! If you don't sense the awe and the joy, then something is not being done right.
The phrase "to pull out all the stops" has its origin in the fact that pipe organs have controls for the air flow, that can "stop" the sound. Pulling out a stop means to allow the sound to flow through a particular pipe or set of pipes. Pulling out all the stops means to use EVERY pipe on the organ. This is what happens at the Easter Vigil.
Countless songs, seven readings narrating salvation history from the Old Testament, punctuated by seven psalms, the Exultet (Easter proclamation), incense, flowers, the Easter fire symbolizing the Light of Christ, the construction of the Easter Candle with markers for the wounds of Christ, even the start of the Mass in darkness, with the lights turned on full force just in time for the Alleluia of Easter, and the use of the "Glory to God" which is not spoken or sung during Lent, Baptism of new Catholics and renewal of baptismal promises for all, and finally the joyous celebration of the Eucharist--these are ALL THE STOPS.
Easter Vigil can take hours. I've attended one as a RCIA team member, several as a musician, and one as a member of the assembly--where I also plan to be this Easter. This will be the third Easter Vigil our family attends, and the second where I've had the privilege of sitting with my family as I am not a part of the choir that is ministering at this Mass.
People say that this one is for the die-hards because of its length, but I think everyone should attend it just once, and in a reverent setting, and with an open mind. The joy will grab you, and you'll want to come back. And seriously--Jesus gave His LIFE for you. Can't you give him two or three hours on one Saturday night of the year?