It is of course the sticky-spamming demo which is the hardest to use!
JUST KIDDING.
I would agree in the sense that competitive spy is so hard, but if you are not in competitive, I would say the scout is rough, no other FPS has the ability to run that fast constantly and shoot things, and double/triple jump. After scout I would say pyro is hard, due to, as previously stated, the airblasting of projectiles and juggling and all that jazz. Followed by Engineer, due to knowing how to keep your base while still being offensive, not to mention actually building a good base in a good spot in the right order for the situation. After that goes soldier for me, high true skill ceiling. Then goes the almighty heavy, you gotta have good spin-up control and good scan-aim. Next comes the almighty demo, gotta know how to arc your explosives and gib people. Then the medic, so you know how to stay alive and keep your target alive, and make best use of ubers n such. Last being the Sniper, which is odd, due to my terrible sniper skills, but that is because I lack the practiced reflexes of point n click, and I love to throw my jarate and get myself killed too much. Anyway, that be my partially thought-out response to class-difficulty order. Yay!