There is no coin can challenge bitcoin right now as you can see in the market value the difference of the huge value gap between bitcoin and altcoins that even you combined all of the altcoins still it cannot compete to the bitcoin. Even the gold value was overcome by the bitcoin into the top and that is why lot of alcoin are now existing because they want to follow the path of bitcoin but i think it is very difficult task for them but maybe if they will try to group together and apply the pump and dump way.