I'll preface by stating that I am an atheist, so take this with a pinch of salt.

My favourite heresy is Arianism, which always seemed far more sensible than the opposing view that Jesus was both born and also always existed. That is logical nonsense. What is often forgotten is that prior to the Council of Nicea this was the generally accepted view.

My second favourite heresy is that God is not the only god. The Old Testament has many references to other gods than Yahweh.