The pyro is underpowered.
As a pyro who frequently comes on top of the list, I disagree. The heavy is too slow, the chargedemo can only hit one player per swing, and lost the advantage of jumping to unlikely ambush spots.
The pyro when played as a spy works best, attack a group from behind and spread havoc. It's also the best sentry-defense class, as it can blast uebered people away, blast/shoot stickies and spyhunt. Only against sentries that are not just-around-a-corner they're really weak.
Or anywhere remotely away from close range. I still love playing the pyro, but every time you look at an ability he has you can see another class does it better. Everybody does well when you ambush, but the explosives classes do better because they can inflict all their damage at once at a range. Pyros can try to run behind enemy lines, but without any added mobility(No projectile jumping, tools to make him less noticeable, double jumping) they are basically doing what you could do with any other class except Heavy and Soldier.
Now Airblast is a very powerful tool, but while it can foil the sticky carpet and block the occasional projectile, other explosives can dislodge stickies that are on the walls and ceiling that they pyro can't see without being in danger of blowing up himself and the hit detection on the flamethrower is iffy. There is a problem when you try to burn somebody running to no effect because they are just out of range yet your airblast pushes them away. And don't get me started on the highly variable flamethrower hit detection range.
I do love playing Pyro, but I just see other classes doing what it does better. Although mayhaps the pyro is a jack of all trades type class when it comes to weaknesses and strengths. But even then the lack of particularly amazing abilities foils that idea.