I love just about every horse out there, regardless of age, gender, height, weight, attitude or image.
That said, I've worked, handled, fed, groomed, and played with 60+ horses, riding about half of them in my horse time. So far all my favorites (with one exception) have been mares, usually bossy mares who make me work for their respect. I love the challenge, and I love that they hold such a high regard for who they completely trust. Both of my lifetime "bonded" mares were very unruly and difficult to handle in the beginning, and I really enjoyed going through the trials and work needed to gain their trust and trust for them because the aftermath was completely worth it. Most dependable animal I've ever had the greatest pleasure of being with.
Although I won't disregard geldings, I think their big ol goofballs and love them to death and really enjoy watching some of the spunky ones play with foals in the pasture. But for my personal horse I just have this inborn connection with a dominant mare with attitude