Many people usually say a song considered their favorite because it brings back memories or they just like it.
In the most personal if you ask me I think that this really does not exist.
Why?
Because simply a song replaces one that is to say there is a point that a certain "favorite" song bores of so much listening and repeating it at all times and in certain cases even irritating it becomes. Until a new song arrives which we like more and another cycle begins until we are boring songs and listening to other new songs that we like.
Personally I think the question would be, what is your favorite current song ?.
In my opinion obviously, greetings