I think they all have good potential, it really just depends how you use them. Humans make good main characters, since people can relate most readily to them. I think the Exo are really interesting, and I plan on putting an Exo character in my story. I'm going to shy away from the Awoken simply because I feel like Bungie will soon release more details about them, which might contradict things I'd create. I personally love the Awoken though, in fact, my main is an Awoken Warlock.