I would like Aronian.
Aronian is the most consistent 2800 player after Kasparov and Carlsen. He has maintained 2800 for years. Not even Kramnik or Topalov can do that.
I prefered Caruana to step up the plate (he is the same generation as Carlsen), but since he did not qualify, and his results are always inconsistent, Aronian is my next choice. Although Aronian never fairs well against Carlsen, and I do not think he will do well in match play conditions.....
In the next couple of years, if Wei Yi or one of the young guns continue to improve, one of them should be the best choice to take on Carlsen.
At the cost of jumping the gun, assuming Carlsen wins the ongoing 2013 WCC , whom would you like to challenge him in 2014? Do give a brief justification as well why you think he would be the right candidate to give Magnus a serious fight.
The candidates have been finalized. http://en.wikipedia.org/wiki/World_Chess_Championship_2014
1. Anand
2. Kramnik
3. Andreikin
4. Topalov
5. Mamedyarov
6. Aronian
7. Karjakin
8. Svidler