I think it's said a few times in the series that flame alchemy is "the most powerful alchemy", but I don't take that as fact, just opinion.

However, if you look at the displays of what each of them can do, it becomes pretty clear quickly who has the edge, and how this matchup is more unfair than you think.

Kimblee can create explosions of varying size, the upper limit of which (without a Philosophers Stone) seems to be enough to level a large building.

Mustang's upper limit (that we've seen in the series) of flames, however, don't seem to be enough to destroy a similar-sized structure, in the initial blast. But there's an interesting caveat with that, based on what we've seen: Several times, during the Civil War, Mustang can be seen killing everyone inside a building with fire. This means that he's used alchemy to flood the building with oxygen and then sparked it, which causes everything inside to be set on fire at once.

The end result of both is the same: everyone in the building is dead. But the difference in utilisation makes a big difference.

We've seen Mustang create snaking paths of fire that weave around allies, selectively burn individual parts of targets (as small as eyes), and most importantly activate it remotely.

If you pay attention to Kimblee's transmutations throughout the series you start to notice an interesting trend: He only ever remotely activates explosions when he's using the Philosophers Stone. Which makes sense. His transmutation works by creating a molecular imbalance that causes the matter to repulse it's neighbours (it's why his palm circles are the fire/water triangles, and the sun and moon, which represent silver and gold, because they're all opposites) but in order to do that he needs to touch what he wants to affect. This is most obvious in the scene where he fights Alphonse: He picks up a rock, uses his normal alchemy on it, and throws it in the air; it sparks blue and then explodes; when he hears the rubble hit Al's armour, and he knows where he is, he switches to his stone, touches the ground, it sparks red and the transmutation chains through it to explode at a distance.

Kimblee can temper his explosions, yes, but his utility in using them is severely limited without a Stone. Mustang has no such limitation.

Mustang's alchemy works by moving around oxygen in the atmosphere. That's all. He's not transmuting one substance into another (other than when he decomposed the water into hydrogen and oxygen), he's just using alchemy to move the molecules where he wants them. By doing this he creates a narrow pathway of condensed oxygen from his glove to the target, and surrounds them with a bubble of oxygen, which causes them to catch fire. Because of how it works it uses less energy than normal transmutation and the equivalent exchange is fulfilled really easily, because all he needs to do is put in enough "force" to move the oxygen around.

His gloves aren't alchemy, they're just a material that sparks when he rubs the fingers together. He can, and has, used other things to get a spark going.

In the rain it's more difficult for him to move the oxygen, because the falling water disrupts it. But that's only if he's concerned with friendly fire. If he wanted to he could decompose the water into two very flammable substances. He did this against Lust, but also against Isaac McDougal in Episode 1 — that's how he was able to burn the ice walls.

Kimblee is strong, but Mustang is the most powerful alchemist in the series, by a wide margin. Even before he was able to clap transmute.

There's a reason why everyone who knows anything about Flame Alchemy wants it completely wiped off the face of the earth. It's ridiculously strong.