I would choose Stannis. He has made it clear he will do what is "best for the realm". Although I likely wouldn't agree with him on many things, he would certainly get things done. Unlike Cersei, he wouldn't put the Sparrows in charge, but he would listen to them, a la the witch.
Snow is too inexperienced and emotional. His decision to go North was not out of necessity against the White's, it was pure emotional need/longing for his dead love.
I don't think Vaerys, nor Little Finger have interest in actually ruling. They are perfectly suited behind the scenes, and would be valuable as such. Tyrion falls in to this same role, a great advisor, but not meant to be the leader.
Arya is a child and I'm not sure why included over many other characters. I would choose Doran Martell at this point over any other than Stannis.
Daenarys may get there, she is incredibly idealistic, but still too emotional. Her + Varys + Tyrion would be extremely formidable, and I would choose that trio over Stannis.