based on ceilings? i dont like a 1-9 scale since a lot of the classes mechanically come down to the same stuff or require about the same amount of skill.
hardest to master = 1, easiest to master = 4;
1. spy, sniper // hitting headshots on people in the air or on people in tight spaces is roughly on par with the map knowledge you need to play spy effectively
2. pyro // i feel like hitting reflects constantly takes a lot of time to master and getting to a point where you can reflect anything is pretty hard. however, since there isn't a whole lot of precision aiming with reflection its lower than spy and sniper but higher than demo/soldier since they are not your rockets, you do not gain all the positives of being able to create rockets for mobility purposes.
3. medic demo soldier // soldier and demo are basically the same class once you consider that the hardest thing to do is maybe airshotting someone or syncing your rockets // medic is about knowing when to pop and not dropping your uber, on top of long distance prediction for the crossbow
4. heavy engi scout // all hitscan classes, engineer has a few advanced mechanics but nothing too complicated. for all three you have to be aware of your surroundings and know how to aim, not too difficult to master and nothing too advanced. if you can track, you can track. if you can track well, why are you playing sniper or spy?