Mares, hands down. I just like them. Every gelding that's been at my barn as been a pig in the stall. Mares are so much cleaner. I like the witchy-ness that sometimes comes over them as well.
Oddly ... I much prefer male neutered dogs to female dogs. Go figure ;)
Interesting, because I prefer all female animals except when it comes to horses.
Geldings all the way for horses, and female animals for everything else. I've known some disgustingly piggy mares, so it's the individual animal and not the gender that makes them messy or not.
My dog is a female Great Dane, and I love that 'make me' attitude she has when she objects to something!
My now deceased gelding was a Queen Beeyotch, and I always said I'd put him up against the ****iest mare there was, and he'd probably win! Nasty, opinionated little ******, but I loved him with all my heart.
He'd try my last nerve, and then do something that brought out his bravery, loyalty, and willingness to follow me through hell.
An animal like that is priceless, and has little or nothing to do with gender.