I love the simple clarity of the first and third, and the balance is very nice. The blacked out background contrasts very well and brings out the small details in the horse. I especially like its effect on the horse's grey coat.
The second's balance is a tad skewed to the right, I'm wondering if it's the hand that throws it off a bit. The colors are really nice, but I'd probably like to see the camera/photo a bit more focused on the horse's face as it's kind of blurry while the mane is sharp, and with the eye closed, the attention isn't really drawn towards the horse for me. I would probably mess around a bit with cropping/ rotating/ blurring and sharpening a bit, just to see. It's a nice picture, though, I'm being nit-picky. :)
Overall, I think my favorite would be the first, though. The blue hue makes him look statuesque, I really like that picture. Sometimes bridles and halters and stuff can get in the way and take the focus off the horse, but I really like them in this picture- it adds to it. I really like the third, too, but I like the blue-ish color in the first better than the orange-ish hue in the third.