Hard to describe, suppose your favourite will just be the one you're attracted to most.
As someone that's pretty evenly between both, they're very different and have unique traits that make each one extremely special. To me there's nothing more erotic and "hot" than sex with a mare, their size, textures, warmth, smells, proportions etc. But equally there's nothing in the world I love more than kissy, cuddly, bed sex with my doggy girl on a rainy day.
Being able to pick her up and have her ride me in cowgirl or various other positions is something you can only dream of with a mare although I'm sure being sat on by 500kg of sexiness would be a happy death, lol.
But I still love both equally and could never choose one over the other.