In my opinion a great cover needs a great photo that is not only clear but also utilizing nice contrasting colours that make it stand out. E.g. the black Friesian against the white snow is perfect contrast. I also think it's really neat that the title is incorporated into the photo. I.e. that the horse's ears go over the text, etc. Anyway, my favourite cover is a toss up between January and July. My least favourite is April, closely followed by June.
All the covers are great. I like the way the "headlines" are on the cover-that piques my interest in what is inside-it also helps me find a "lost" article when I look for articles on certain subjects.