This is a tough one, because I don't really have a favorite subject. There is no music and arts department and they don't do things like take an English class, so that leaves out my real world favorite subjects.
McGonagall is my favorite professor, but that doesn't mean Transfiguration is my favorite subject. I think I'd have to go with Charms taught by Professor Filius Flitwick.
Why Charms? I'm not really sure, to be honest. I don't care for chemistry or cooking, which I think potions would be like. I think Transfiguration might be frustrating. I can see it as fun, but something that would take a long time for me to master that I might not enjoy it as much. I think Defense Against the Dark Arts could be fun. Maybe I'd like that instead. But, I know I wouldn't like the revolving door of professors especially getting certain ones like Lockhart, Umbridge, and Snape, as well as learning about the three unforgivables in "Moody"'s class, so I wouldn't label that one a favorite.
That leaves, what? Care of Magical Creatures, Arithmancy, Astronomy, Ancient Runes, History of Magic, Muggle Studies, and Charms.
I like animals, but not the crazy ones like Flobberworms and Blast-Ended Skrewts that Hagrid introduces in his Care of Magical Creatures class. Math and foreign languages are not my thing, so that would rule out Arithmancy and Ancient Runes. I took Astronomy in college and while the class at Hogwarts is different, I'd probably be just as not good at it as I was then (I made a C, my first C in college). History of Magic is taught by Professor Binns and I've read he's extremely boring (and dead) so I'd probably have a hard time staying awake. I'd probably find Muggle Studies mundane and too easy or more like history which is not a subject I enjoyed in school.
That leaves Charms. Flitwick is a short guy with a squeaky voice. He seems like a really cool guy and he's the head of Ravenclaw. I think his class would be fun.