FREE - In Google Play
FREE - in Win Phone Store
I don't think there's any contender at the moment who has the consistent play to beat Carlsen but I wouldn't be surprised if within a few years someone eventually does.
I'd bet on Wesley So. He's the no.1 threat to Magnus (he is growing up fast, and if he's not better already, he'll be in the near future). Another upcoming serious threat is Mamedyarov.
I think someone like Wei Yi is a good choice for the feature. I dont think anyone will topple him just yet for the next few years. It will be up to one of the youngsters to come up and show their stuff