ESC. My bet would be there are 1 or more blown FETs. In the amount of time it takes for you to plug the battery backwards, get an arc and pull it apart, the damage is done. My advice would be to change the connectors to something like Deans where it's impossible to plug in backwards and there is no chance of this happening again.
http://www.amainhobbies.com/rc-cars-...-wsd1300/p3596