There are two skill plateaus:
0) Nub -- you suck, epically.
1) Proficiency -- you are able to provide what your teammates expect of you.
2) Expert -- you force the other team to rethink their entire strategy.
THE SPY
In terms of getting to proficiency, I'd say that it's gotta be him. I am not one to learn with a manual, so I just kind of played the classes until I understood what they did. I had no idea what the sapper did for the longest time. I learned the hard way that stabbing drops your disguise, that you can't attack while cloaked, that disguising and cloaking and decloaking all take a second to kick in, that being anywhere near a pyro is a death sentence, and that when opponents bump into you they will drop anything they're doing to hunt you down. Valve has even noticed how ridiculous it is to try to play spy passably well and they're reworking how the whole disguise thing works. It's very nuanced. If they wanted to fix spy, in my opinion:
1- let them change their disguised weapon (engie with a wrench or toolbox is a lot less suspicious)
2- let them pretend to shoot at people without damaging anything.
3- let them walk through enemies while uncloaked.
THE SCOUT
But when it comes to reaching Expert level, you have to add Scout to the mix. He'd be a lot more dangerous if he didn't have to actually point his weapons while trying to evade fire at 50mph. He'd live a lot longer if he didn't have the weakest health of all the classes. He'd also get played more often if he had some role that other classes couldn't fill. He can double-jump, but soldiers and demos can go anywhere he can go just as easily and still arrive with more health than him. He can capture twice as fast, but anyone can capture. He's fast, but after you factor in level 3 teleporters, what does that speed really add up to in the end? It's rare for you to hear, "man we could really use another couple of scouts."