I think it's obvious what most people think on this one.
Let's see... With Fire Blast, you get a possibility of dealing 625 base damage if they all hit (5), while with Flamethrower with 100% accuracy, you get a chance of dealing 1,425 base damage. Even when boosted to 8 PP for Fire Blast, 1,000 is the base damage, with only a 70% chance of hitting. And then, you factor in Hit Points and Special Defense...
My choice is Flamethrower because it gets things done and deals a lot more damage in the end.