Nico Rosberg would definitely be my pick. He is fast, I mean, he makes Hamilton look slow, although Hamilton right now has trouble it is still very impressive. Hamilton is after all one of the fastest guys out there.
So, plenty of speed, and he has shown that he is easily capable of winning races if he is given the right car. That is incredibly useful if the lead driver fails for some reason.
I think he has showed an incredibly good consistency all around. He doesn’t seem to be able to deliver above the expectations, but for a number 2 driver, I think, that it’s more important to not deliver below the expectations. And he certainly doesn’t do that very often.
He also showed in Malaysia that he will hold position when needed, and unlike Massa, his performance didn’t suffer noticeably form it.
He is also quite young, which means there isn’t a need to look for a replacement any time soon.
My second pick would be Adrian Sutil. He seems pretty fast and consistent, but just how good he is at the front is hard to say at this point.