My older gelding Midnight likes right under his forelock or behind his front legs (where the girth sits) scratched.
Flicka my mare like it when I rub her white markings on her face. She is also in love with me rubbing the inside of her ears. I have no clue why but she loves it with a passion. She'll lean into you and everything.
My first pony, Rosie (RIP 2009), loved being rubbed right under her eye. It'd put her to sleep everytime.
So where's your horses favorite spot??