I'm rather torn between the pyro and the demoman. Pyro is great for uncovering those pesky spies, and on the offensive side, it's great for weakening defenses. The only downside is the lack of range. Used correctly, the pyro can dominate either side of the game.
Just the same, the demoman is great for taking out sentries when you go on the offense. Those sticky bombs are very powerful, and hidden in the right places, will cause lots of damage. The demoman is also great for taking out intruders to a team's base. Someone sitting on your cap point that you don't like? Kill him with some stickies. And if that fails, start shooting grenades.
However, any way you put it, no class can excel when there is a lack of others.