Hi, my name is Sarcasm, and I like walls of text. A lot. Also, I'm at work and I'm bored.
I rate engie as the easiest class, mostly because in competitive there isn't ALL that much to do. In terms of fighting, you aren't required to. You're generally playing a support role in the same way as a medic, but without having a massive bullseye on your head. You can be aggressive, and learn scout/hitscan dm, but overall its not required. Drop your teleporter at the start of every round, watch as your teammates suicide for you on payloads, build a level 3, and run the wrangler. You start to die, and your sentry dies on payload? Okay, back up to the next point and build there. It requires a small amount of game awareness in the sense that you need to listen for spies, and hear the spy calls. But all in all, you're free to do whatever/go where you please. Unless its gullywash last, in which case your sentry is going on the right or the left, and you're not moving. Sorry.
I'm probably going to get flak for this, but its just what I've seen. I'm putting sniper this far down, because the hardest part of learning sniper, is learning to be consistent. I recently asked one of the better snipers I know, what made him so good. And he said "I dunno, I just shoot as much as possible". The difference is that his skill is ALWAYS consistent. If he's having a bad day, he's going to miss most if not all of his shots. But any other day, he'll land them. You can run forward and quickscope, but your total damage if you are consistent, is higher than any other class. 150 per bullet means you guarantee kill spy/sniper/engineer/scout without buffs. In lower tier, its a lot more common for you to see players that are unbuffed, which means sniper COULD BE a lot better. You also pretty much deny any possibility of jumping as demo and soldier, and you force the heavy back by cutting his hp in half. Obviously, a skilled sniper can destroy a lower tier team, for example, I rang for a silver team to find Axiomatic, a plat sniper main, playing in silver. For the first few rounds, we gave up a total of 60 headshots, 4 of which were me. The next half, we focused him entirely and brought his total headshots down to 24. You cannot as a sniper stop a soldier scout combo from killing you, if they are allowed to. So base skill of sniper can APPEAR high, but all it is, is consistency. Just sayin.
I put heavy higher than sniper, because if you think about it in a competitive sense, heavy is probably one of the more important classes. The reason why in pub games he's considered "op", is because people don't know how to deal with him. A soldier can juke around corners and fire 4 rockets, and he's dead. If he has a medic, you kill the medic first, then kite backwards. Its not that hard. In competitive, you juke around that corner, from the heavy? The scout is waiting for you, because you've added comms. The heavy influences how the game is played. Long range harass isn't beneficial in terms of damage, but it scares people. Ubercharged heavy doesn't do much except provide a brick for the demoman to hide behind, but his positioning is crucial. Also, in a lot of cases, the heavy can be a main caller. He's the one who can respond to a scout flanking your medic, or a soldier jumping, and completely cancel the jump. Tracking as heavy is a big deal, you can kill a scout easily, if you can hit him. You can knock the sniper's aim off, by shooting at him and giving him a small bit of recoil. Overall, you need to know WHEN to push, WHERE to push, and when to pull out. You need to be able to block damage for your medic, and literally do everything to keep him alive. And that isn't as easy as a lot of people think. While heavy's skill floor is fairly low (lol, I left click and kill things), his skill ceiling is rather high. (Ever watched polk play heavy?)
Pyro. Mmmmm pyro. On tuesday, I played process, we pushed out to mid at the normal time, pushed out towards our sewers, and their IT. Their soldier jumped directly for me. Our pyro reflected the rocket meant for me, airshot the soldier, pulled out the reserve shooter, and finished the soldier off in mid air. Thats not something super easy to do. Its not an easy class as a whole. The basic concept of puff and sting is. But there's more to it than that. Knowing exactly where the spy will be, knowing every location the soldier can bomb from, same with demo. Expecting those kinds of things. Stuffing an uber while your medic buffs you. Knowing you can clear an entire team off of a point to force a capture. Coming up from water in gullywash to knock the entire team and all the stickies off the point to solo cap. Knowing where most demomen put traps, and taking the brunt of the damage if you screw up. Being the person who goes back and gets your medic because he died on koth maps. These are NOT easy tasks to do. A sniper doesn't need to worry about these things. In fact, the only class that needs to worry about MORE things than the pyro, is the medic. I'm not scared of spies anymore, because I know my pyro is competent. And it shows. Per match, on average the spy gets me 2-3 times. Versus a pyro who doesn't know what he's doing, my average would be 4-6. I'd rather keep half those lives, and half those ubers. So, I give thanks to the pyro mains who actually focus on keeping the medic alive, instead of deciding its more enjoyable to roam.
Medic is tough, but only in the sense that you have to understand what each class does, and in 90% of games you have to be able to call and position yourself so you don't die. Calling uber times and comms are like, literally imperative to being a good medic. The hardest part of medic is applying the theory to the actual game, and even if you've run the map a million times, there's always going to be that soldier from out of nowhere. Adaptation is huge, and in higher tiers, the teams look to the medic for where you want to go. THAT BEING SAID. I feel its still in 5th. Because there are more to other classes in actual games, than there is for the medic.
In terms of spy, a lot of said "Skill", doesn't come from how many frags they get in higher divisions. It comes from comms, again. If you're a spy running cloak and dagger, and following the enemy combo around, you can call where they're pushing, and what they're doing. Which can literally save your team, and allow you to win the game. Most of what spies do is ambassador sniper (Now diamondback shoot), and go for key picks when there is focus being pulled away from them. (Yes, this isn't in pubs. I've literally seen a spy disguise in front of an enemy pyro, and have the pyro walk away. I don't put bearing on skill levels from pubs.).
I put scout at 3rd, because if you look at scout in sixes and highlander, the role of the scout is based a lot on how the team works with them. With the scout in highlander recieving 185 hp as much as possible, he can do work, sure. But he gets shut down a lot. You can go practice DM for a thousand hours, and be fantastic at it. But you still can't kill a full combo unless the combo doesn't know how to track. You can pubstomp as scout, til a half decent demo comes up, and drops a sticky and a pill on you, and you die. Its the same in HL. For 6's, you have a partner, but you're also directly going against the other two scouts, and its a dm matchup. That being said, playing against two splash classes isn't gonna be much fun, unless you can bounce around them. If they're any good at tracking, you're gonna lose.
For demo, he's second, just because of specific rollouts being pretty rough, and the fact that its yet another element people need to get good at (Hi jedders). Look for example at the process fast rollout for demoman, or the badlands valley rollout. You have to be good at rollouts, which means you need to understand airstrafing, how to control sticks, jump times etc. But you also have to be able to anticipate where sticks need to land to hit moving targets. I recently played against SS, and invite player who played demo in highlander, and it was interesting to see how different his skill level was versus the skill level of an average gold demo. The fact he was able to anticipate my random movements, and land literally EVERY sticky on me is not something you can pass off. On top of that, you add yet another projectile weapon with pills. So you have to not only be good at a delayed projectile, but also one that explodes as soon as you hit. The speeds on both weapons are different and its once again more of adaptation. There are other parts to demo that add more difficulty, but those come with unlocks. (E.G. Pain train).
The reason why I put soldier in first, is because of how much bullshit you have to deal with as one. Take highlander for example. You get to bomb the medic, while also having to deal with the sniper/pyro/heavy that are all trying to kill you. At high levels, you need to be able to do some wicked jumps (Jumping off both obelisks in lakeside to land behind them), you need to be good at hitscan weapons (shotgun), and projectiles at the same time. All without generally getting much buff, at high tiers. My soldier on my team is generally happy with getting 250 HP per jump. And he still does work with that. In terms of sixes, there's two different ways to play soldier. You either have to be good at keeping your medic alive, or killing the other medic. Both require very different skillsets. You need to be able to rollout, without heals, and bomb specific targets, without heals. Which means you need to know every map, and know how everyone moves on each map. Being able to airshot habitually is something thats difficult on soldier. Knowing when to use different weapons, and being able to move players with rockets is harder than people think. Yeah you can "Shoot the ground and win", but that doesn't get you anywhere when the other soldier just got a juggle and then a double airshot on you. Go surf off other people's rockets and then wall jump to kill their med, and tell me soldier is easy at high levels.
TL;DR
Engineer/sniper/heavy/pyro/medic/spy/scout/demo/soldier.