The Catholic church, by far. Catholicism is the original Christian church. The apostle St. Peter being the first Pope when Jesus said to Peter, "You are Peter, and upon this rock I will build My church". Peter was in charge of the church following the ascension, and when he was martyred, his successor St. Linus became the Pope, and so on all the way to the present, Pope Benedict XVI.
To put it this way, the Catholic church has been around for about 2,000 years. We can trace our founding all the way back to Jesus himself.
Catholicism was founded by God, and the Church was born from the side of Christ when it was pierced by a sword while He hung dead on the cross. The Anabaptists, a sect which rejected their baptism, was founded in the 17th century, so roughly 1,700 years after the Catholic Church was formed by Jesus Christ and the Holy Spirit.