Flamethrower. It has more accuracy and PP, and I've always hated Overheat for some reason. What's the use of an attack that does like half the amount of damage the next turn? Unless you switch then come back again, which makes it useless if you don't KO in the first turn.
Eruption isn't that good as well, you need a fast pokemon to do it. Camerupt is so slow that a pokemon will damage it before it can attack, so Eruption will do very little damage. Fire blast is too inaccurate and has too little PP to be good.