Flamethrower is the ultimate in fire-type moves, and has been for a very long time. True, there are stronger moves out there, but none are as accurate or long-lasting as Flamethrower. Any fire-type that wants to reliably land a lot of one-hit KOs using their special attack stat should know Flamethrower.
Finding Flamethrower isn't too difficult, but it also can't be achieved until near the end of the game. You need to have defeated the plans of Team Plasma in the Great Chasm and made your way to the beginning of Victory Road on Route 23, just before the badge checkpoint that allows you access to the road.
There are two houses on Route 23. The first allows you to heal up your pokemon, which is handy as many of the trainers in this area are relatively tough. The second, closer to Victory Road itself, is home to an old man who, when addressed, will hand over TM35. Use it to teach your pokemon Flamethrower.