First off, let's talk about starting rounds. I see this all the time. People either don't disguise or disguise as an enemy and try to hide. Do you know how the Disguise Kit works? There's a key (Default "-") to change all disguises to allies, which will help you go unnoticed.
This only helps if you're using the DR and trying to feign to get across the line. Because, surprise surprise, people will attack enemies and if you're disguised as an ally they see you as an enemy.
For a second note on the Disguise Kit - When using the Dead Ringer, I see a lot of people either a. Disguise as an enemy and let themselves die without even attacking once, or
Sometimes there's actually a good reason to do this. There's nothing more satisfying than letting your opponents know you're still alive and that they need to keep their guard up. I love having 3-4 players chasing me all over the map, wasting their time, because I've made an obvious feign.
Furthermore, I see people attacking the wrong targets. Your order of priority should go like this: Engineers (And their nests), Medics, Snipers, Heavies, Demos, Spies, Soldiers, and at the bottom, Scouts and Pyros. Try to stay away from those last two. Try to always go for the top three - You are a support class. Your goal is to help your team. This means getting rid of Medic ubers and good Snipers, as well as clearing Engineer nests for the final push.* All other enemies you should only attack if they seem to be in front of you with their back in your face.
This is such misguided advice.
There are a lot of ways to play spy and depending on how you're playing, or what your team needs, you should be switching up your targets. An engineer who is new to the game is not nearly as much of a threat as a black Box soldier who knows what they're doing.
When your entire team is having a problem with a single enemy in particular, often that's what your focus should be. find where your help is most effective.
Alternatively, if you're playing gun spy, you may play Spy as a flanking class. With the DR, you can use your gun to seriously shut down an alternative route when you have a soldier/scout with you and you can use the loud decloak to distract your enemy when you're behind them, but they have a soldier in front. Better players really react to the sound, so you can often save your team mates by forcing them to turn around.
A pick-order list does help you prioritise targets in general, but the truth is you should never focus a class simply because they're high on the list. You should be focusing on being effective. If their Sniper can't headshot, he's really not your target. If their medic is a battle medic, don't give up a good position to kill them. Choose your target based on how they're helping their team, not on some pre-ordained list that some guy on the internet told you.
I often see people choosing the wrong revolvers for the wrong situations, for the wrong reasons. Pretty much every Spy should have an Ambassador - This gun is extremely powerful and should be used to its maximum potential. I see people using the Enforcer because "It's good", but really, the Ambassador is better in all situations. You may just go "But Shonoun, I can't aim for shit!", but, well, that's your own fault.
Wow, you're one of those 'git gud' pricks. The ambassador is a great tool, but that doesn't mean it's for everyone. I'm a revolver spy, the revolver is a very powerful tool. A lot of great spies use it equally with the ambassador (Acooma, Noko, Khazul, Carlotso, MS, etc). I cannot count the amount of spies who are surprised when I can almost always out-DM them, despite them being exceptionally good with the Amby.
Go get tr_walkway (Newest version, whichever one that is). Right now. Like, seriously. Do a google search. This map is ESSENTIAL for all Spies. It will let you practice every single killing skill you can have. Rampstabbing, Jumpstabbing, Ambassador training, Matador Stabbing,
…I don't think you know how matador stabbing works. The bots on tr_walkway don't react and the entire point of a matador is to use a player's reactions against them to get behind them. No reaction, no point.
Most other revolvers you should really avoid. The L'etranger is good in some cases, but it is extremely situational and that damage loss can fuck up your game. Be careful when choosing a revolver other than the Amby.
I don't think you understand how effective the L'stranger can be. Again, there are a lot of top spies who will use the L'stranger a lot because of what it allows them to do. I use it a fair amount as well, it's a useful tool. The L'Etranger is a tool that allows you to position yourself for the most effective pick. Not the most picks, but the most effective one. It doesn't matter how good your amby aim is if you can't get into the right position and being in the right place is half the battle.
Anyways, I got a bit carried off with that revolver rant. Sorry about that. Onto dealing with Engineers. Well, I can't explain it in much detail here. See footnote* (Haha you really expected me to talk about stuff here, well, sorry to dissapoint).
This entire rant is a disappointment.
People often disguise as Scouts or Medics, who can easily be differentiated from real players due to speed differences and general behavior.
The scout can be an excellent disguise when you're using the invoice watch due to the smaller player model. There are a lot of spots that you can decloak disguised as a scout that you couldn't disguised as any other class.
I usually go for Pyro or Demo, their speed is normal
The Spy-Pyro is super easy to spot due to the lack of flames. If you're talking about behavioural disguises, I would never recommend the Pyro. The Demoman walks at 90% speed, so always take this into account - you will fall behind when trying to catch up with someone, even if you take a marginally shorter route.
Engineers aren't usually seen at the front lines, and when you go to the back lines, you're 100% going to run into the very Engi you're disguised as.
Though you could definitely do worse on KotH maps than disguising as an engineer, they end up playing a flanking role on KotH, so you can be anywhere as an engineer and not look too suspicious.
Disguising as an enemy Spy is very tricky. For some unknown reason, people tend to spycheck friendly Spies more than any other class, even though Spy is an uncommon disguise choice.
Spy is the most common disguise choice, which is why people psychic it. Alongside that, people look at the spy and are reminded that spies exist and so remember to psychic. The spy can be an effective choice sometimes when crossing the lines. If your team psychic you and you cross the line at that time, often it looks like you were running away - you can use this to your advantage to not look suspicious and get a couple of nice stabs.
Generally speaking though, you should be switching up your disguises. Change your disguise every time you cloak or have a little bit of down time, keep your enemies guessing as to which class you're disguised as. Don't rely on a single class to disguise as or you'll just get spy checked more often.
This post wasn't entirely devoid of good points, but I don't think that this addresses a lot of the real problems that spies have nowadays. The whole thing was a bit painful to read.