What is the single biggest reason you believe in geocentrism?

The biggest reason is that Scripture teaches it. Scripture is the inerrant, inspired word of God, every single word, so if Scripture says the Earth does not move and the sun does, and that the sun moves around the earth, I take that as God himself standing in front of me and saying the Earth is fixed and the sun and stars go around it every day. Second, you have the testimony of the Fathers; Leo XIII in Providentissimus Deus number 14, in 1893, says that when the Fathers are in consensus on a matter of faith and morals we are required to believe it as doctrine, and there is no more pristine consensus of the Fathers than geocentrism, with baptismal regeneration a close second; not one Father was against it. Third, you have the magisterium: every time geocentrism was challenged, by Nicholas of Cusa, Copernicus, Foscarini, and Galileo, the Church defended it, and Paul V's eleven qualifiers declared heliocentrism a formal heresy because it goes against the inerrant word of God. And finally, science itself, once Einstein showed that Newton had wrongly ignored the rest of the universe, has come to admit that a fixed Earth with everything revolving around it is possible and cannot be denied. So with Scripture, the Fathers, the magisterium, and now science, you have everything you need; the only thing lacking is courage.