Which disguise should I pick as Spy?

Unless I'm chasing Engineer buildings or a specific player, I practically select the disguise at random to save time, avoiding slower classes. Is it possible to make informed disguising decisions at all times?

While disguises may not fool experienced players, they are mandatory to spoof Engineer's turrets.

3 Answers

Pros and Cons for disguising as each class.

Scout

PROS: Changes your hitbox, harder to headshot you.

CONS: Does not speed you up in any way, and you can't double jump. This shows the enemy immediately that it's not a Scout.

OVERALL: Would not recommend - the enemy expects you to shoot, double jump and sprint, none of which you can actually do while disguised.

Soldier

PROS: You don't need to have a degree in method acting to do well in this, just walk along with the enemy team, scream "MEDIC" every once in a while, and don't run into your team, because they would expect you to shoot.

CONS: Makes you slower, and if you are disguising as an experienced player, the enemy might expect you to rocket jump and get suspicious if you don't.

OVERALL: Good for some players. Depends on your playstyle.

Pyro

PROS: Easy as hell, and doesn't slow you down at all. You can just act natural, as if you are actually playing Pyro.

CONS: You can't backstab very well, as a Pyro is expected to be the frontmost person in the enemy line of defense, and if a Pyro is hanging back behind, the enemy will get suspicious

OVERALL: Good, but only for assassination - don't try chainstabbing with this.

Demoman

PROS: Doesn't slow you, you can stick behind the team, don't need to do much to look realistic.

CONS: Experienced players will expect you to charge, or sticky jump. If you are disguised as someone who looks relatively new, they may pass, but it is risky.

OVERALL: Great, but once again, depends on your playstyle.

Heavy

PROS: Nobody suspects you, ever, because nobody ever disguises as heavy.

CONS: Where do I begin. Movement speed, frontline, easy to headshot as the heavy's neck, a very easy target, is right where your head is.

OVERALL: Depends on your skill level. If you can handle the cons, then the pros outweigh them.

Engineer

PROS: Amazing on Defense. A Spy disguised as Engineer would never be checked if he is near a sentry. Also, you can easily stay next to other engies, without any doubt. Then you can sap their crap and stab them.

CONS: Needs to look realistic - takes practice and a bit of method acting.

OVERALL: Good if the enemies are Defending, meh if they are attacking.

Medic

PROS: People with low health run to you like dogs to the hunt, and you can easily finish them off with an easy facestab or an Ambassador headshot.

CONS: If you don't heal people, they might check you, which is bad.

OVERALL: Once again, depends on skill.

Sniper

PROS: If you're in a Sniper spot, your disguise has their primary weapon out, and you're crouching, nobody ever checks you, unless they're a W+m1. Method acting at its finest.

CONS: Hard to get to a sniper spot without being checked - an out-of-place sniper is very rare in TF2.

OVERALL: Good, but you may need a safe place to uncloak when getting to the Sniper spot.

Spy

PROS: Very efficient if you know how to use it; you can be behind the enemy team, not many people will spycheck you - and if you uncloak in front of the enemy, they won't be too scared.

CONS: Hard as hell to play off well.

OVERALL: Skill-dependent.

All 9 disguises can have use depending on a large variety of situations and playstyles

But you should never assume that your disguise is going to keep you safe. In fact, you should always prepare for the worst happening (being spy-checked) and have a Plan B ready to go.

Some of the basics for disguises can be answered with some simple questions:

Does my disguise make sense for where I am on the map right now?
Will I run into the person I am disguised as if I stick around here / take this path?
Should my disguise fool the enemy this time, do I keep the same disguise every time I go to a certain spot?

After you get the feel for how the enemy plays and after playing Spy yourself for a while, you can get pretty good with your disguises.

A BIG part of disguising that a lot of newer spies tend to not know is the fact that you can disguise as TEAMMATES as well as enemies. This can be paired with the Dead Ringer to create more believable "fake deaths" as if it looks like the enemy killed a Soldier, they may not think that it was a Dead Ringer Spy. Another possible use of teammate disguises can be used to lurer enemies in for trickstabs once you learn how to do them.

All in all, disguises should be used carefully, as there is definitely more to it than just disguising as a certain class just because it feels right.

The answer provided by Amitai does also include some nice facts about each disguise, I would recommend reading those as well :)

I disagree with some of the points made in the other answers so I'm adding this one.

When you haven't yet gotten into the enemy's turf, be disguised as either a teammate or an enemy Spy so the enemy doesn't know your team has a Spy. The former is good for Dead Ringer use and baiting enemies around corners and such, while the latter is more likely to be ignored by enemies at a glance. You should probably try not to be disguised as a teammate who's nearby if their cosmetics are too loud.

Amongst the enemy, there are three groups of disguises.

Common: Pyro, Demoman, Engineer, Sniper These classes have reason to be wandering amongst their own team for long periods of time, making them an ideal choice of disguise. Of course, this can lead to players being aware of this, and being more likely to spy-check them.
Uncommon: Scout, Spy The Scout disguise is very poor at fooling people who are paying attention, but those who take one glance at a distance could be fooled and therefore assume the Scout is real. There is no mechanical reason to doubt a Spy disguise, just your behaviour, but the randomly-generated information displayed as to who your disguise is disguised as can give you away if it doesn't make sense. In addition, some players instinctively fire at all Spies regardless of colour. Combined, these two classes are rarer disguise choices mostly because of their downfalls.
Situational: Soldier, Heavy, Medic These disguises are the rarest to see and so the least likely to be suspected - but only because they are very weak in most other areas. Soldiers and Heavies can easily fall in with the team and get a Medic attached to them without needing to do much, especially if they end up disguised with low health, but they cannot chase targets and will have a hard time not blocking enemies. The Medic disguise has zero believeability as he cannot heal at all times as expected and always shows 0% übercharge, but this itself can led into baiting enemies for stabs or into your team.

The most important thing about disguise choice is that you remain unpredictable. This is not the same thing as "disguise randomly all the time" - a smart team could realize the enemy Spy never disguises as the same thing twice in a row. If you end up making the enemy think you only disguise as one thing, you can do something else and fool them for a while (though of course this disadvantages you for a bit to start with).