Engie. I become one of the most important members of the team, able to erect sentries to ward off hordes of enemies, dispensers to keep my teammates in top condition, and teleports to field combatants quickly and efficiently. With a single pybro nearby, my sentry will be able to operate in safety from any of those wretched spies. I also know when to use my mini-sentry for its most important yet unspoken purposes: shrouding the minds of my enemies with an irrational amount of anger and frustration!
Pyro. When I become one, I am suddenly able to pin-point spies that could otherwise take out vital buildings, remove strategically important snipers, or even kill medics about to deploy ubers that would've made/kept the current battleground in our favor. I'll also be able to airblast away enemy ubers, rendering them near useless, likely sowing the seeds of distrust among the other team, leading them to question the effectiveness of their medic or the one who received the charge. I make use of the lowly homewrecker, I lack the Maul and Neon sign thing, to give engineers as much help as possible, saving them from frantic, and possibly in vain, rushes to quickly remove a sapper from their failing creations.
Spy. I will admit, my weapon combination is not exactly unique. I make use of the Spycicle and Dead Ringer. This enables me to move behind their lines undetected. In the event I am detected and without any hope of escape, I will attempt to attract as much attention to me as possible, keeping the enemy busy and away from the front lines, where my teammates will be able to complete their objective with a little more ease. Should I have my cloaking device and melee weapon prepared, I will attempt to stay at the front of any charges I take part in, as I see myself as a useful meat shield, which could either block the attacks of a sentry while an ubered heavy fires on it, or sap it quick enough for someone to immediately deplete its health.