Pros and Cons for disguising as each class.
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.
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.
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.
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.
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.
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.
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.
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.
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.