Upper middle class: Because you've got enough money to be happy, but not enough to be smug.
Oh..by class you meant...riiiiiiiight.
Spy. No other class gives me a genuine adrenaline rush like spy, seeing the back of that heavy, medic, demo and pyro all clustered up right in front of you, moving in for the kill...taking out the medic just about to deploy uber at the start of a round on dustbowl, sap/stabbing the sentry that's been holding your team back...it's really really rewarding.
Engi: Having the best weapon in the game - L3 sentry! It's just fun having that, a well placed dispenser, and teleporters and knowing that you're being really really useful to your team in so many ways (kills, teleportation to save time, dispenser handing out healing, ammo, cloak). An engi's usefulness on the frontlines is unparalleled.