Well, either works, to be honest. I'll give you 3 choices...
Flamethrower: Great move. 95 Power isn't bad at all, and has a 100% accuracy. Go for it if you want strong, consistent damage.
Heat Wave:Great move for double battles and in general. only slightly stronger than Flamethrower, but watch out for that 10% chance of missing.
Fire Blast: Now, if you really want to go all out and dont fear missing every once in a while, go with this. the 120 power is really good, and brings in the KO's.