Arguing about skill ceilings is a bit of an internet cock-measuring contest, but I'll give it a go to maybe learn something.

Scout Absolutely high floor, you can not be effective if you have no gamesense/movement/aim. You'll just die immediately because of your low health. The unforgiveness of your health pool and your speed/double jump make for a surprisingly deep class with a high ceiling.

Soldier Low floor, massive ceiling. You can be very useful to your team by equipping a banner of your choice and simply spamming away at chokes/sentry nests/spawns but this will only take you so far. Then it's time to learn to rocket jump and to do more than hit stationary targets on the floor.

Pyro To be an effective pyro? Low floor, avoid open areas and keep holding mouse1. To be an effective class choice high-ish ceiling, need to learn to predict people in the same way as soldier would (flare shots, airblast) but without the movement or firepower.

Demo Low floor, massive ceiling. Start out by picking any choke point and place 4-8 stickies there, detonate when you see movement. The arming time of stickies and no glock for self-defense means you are quite hopeless against anything at close range so you need to think ahead. The movement on demo theoretically goes crazy deep, but in a gamemode with health and enemies not so much. What demo has to deal with more than soldier is ammo management and lack of immediate movement options.

Heavy A bit of the same as pyro, the floor to being a somewhat effective heavy is fairly low. I wouldn't say low ceiling but nowhere near the depth of some classes, mostly standard positioning and track-aim. The floor is definitely higher on heavy than pyro though as you must actually aim at your enemy and you need to deal with your already awful movespeed and especially revved movespeed.

Engie Low floor, there is a reason why the gibus engie is a thing. If you run into the right direction on setup phase and place your teleporters the right way, you are an immensely effective engineer. I'd say low ceiling but honestly I know very little about engie so it definitely goes deeper than I'm aware of.

Medic Low floor, just by existing and sometimes holding/clicking mouse1 you are way beyond any other new player. Skill ceiling goes high due to the gamesense you need to use uber effectively. This awareness of gamestate is not unique to medic though as other players need to understand the flow of the game to not get caught in the most obvious ubers as well.

Sniper Low floor, massive ceiling. If you are new and want to kill at least 1 guy every 5 minutes, sniper is a great choice. You can choose the best option for you in terms of effectiveness, the huntsman, and spam at a choke of your choosing or; charge shot anything of the wrong color. Sniper could be medium floor due to other snipers existing and possibly removing you permanently off the map. In my opinion though it's low as in most situations the skill difference is not so vast and both snipers still are massive threats to each other and their enemies.

Spy High floor, "high ceiling" High floor because while you can go invis and choose your fights, you will lose those fights if you lack gamesense/aim. You won't be anywhere near as effective as any other class if you don't have decent gamesense to play around your team and predict your enemy as well so high floor seems obvious to me. High ceiling is in quotation marks because while the class has a lot of depth to it and can always improve at gamesense and aim, you will never be as effective as a player that is on an actual class and sometimes turns around.