Disguise

From TF2 Wiki

The Spy is able to disguise as any character class of either team using his disguise kit. When the Spy disguises, he will be surrounded briefly by smoke and the glow of his own team color. Once disguised, the Spy will appear to the enemy team as the selected class and team color while wielding that class's primary weapon. To teammates, he will appear to be wearing a paper mask of the selected class, and his health indicator will note the team and class of his disguise.

If a Spy disguises as a member of the enemy team, he will appear to enemies to have the name of a player on that team of the same class. If there are no players of that class on the team, he will have the name of another random player on that team. While disguised as an enemy, a Spy is able to receive healing from enemy dispensers and Medics. It is even possible to receive an ÜberCharge from an enemy Medic.

Note that enemy dispenses always try to heal you while disguised. This happens while cloaked! A smart Engineer (or any player) can see the team-colored healing line going from the dispenser to the cloaked spy, giving away the spy's location. As a spy, you may want to sometimes change your disguise to your own team to avoid having enemy dispensers reveal you when cloaked.

Disguised Spies are unable to pass through enemy players or taunt, but can still use voice commands: to the Spy himself and to the enemy team, these commands will sound like the disguised class; to teammates, the Spy will sound normal.

While disguised, the Spy will slow down to match the movement speed of his disguised class. This speed reduction does not apply while cloaked. If the class is faster than the Spy (for example: disguised as Scout or Medic), he will move at default Spy speed.

A disguise is automatically lost if a Spy fires his revolver, swings his knife or grabs the enemy Intelligence. Placing sappers will not remove the spy's disguise.

The spy has a keybind to don his last used disguise (default B)

Class Disguise	Effectiveness	Info
Scout	Low	A highly inadvisable disguise. The speed difference between a Scout and a disguised Spy is very noticeable. This combined with the lack of double-jump renders the Scout disguise practically useless.
Soldier	Medium	A Soldier disguise is useful in certain situations. Soldiers are common, so you may draw less attention from spy-checkers. There is also some chance an enemy medic may ÜberCharge them. The movement speed reduction makes backstabs more difficult, as well as making you more vulnerable to spy-checking (especially players running through you).
Pyro	High	One of the most useful disguises. Spies disguised as Pyros move at regular speed and don't look suspicious with their primary weapon equipped. They have an excuse to be hiding in corners, running around enemy lines and standing near sentry guns. Since any Spy should be staying far away from enemy Pyros, enemy players are also less likely to notice any name discrepancies. They are also a potential candidate for an enemy medic's ÜberCharge.
Demoman	High	Demomen move with a negligible decrease in movement speed, often hang around enemy lines and may receive an enemy Medic's ÜberCharge. The Demoman primary weapon is often carrier, and looks like the secondary weapon regardless. However, a Demoman who is not watching sticky bombs while in base is suspicious.
Heavy	Low	Rarely useful. Heavies have little excuse to be hanging around anywhere except the front lines and will be suspicious when not spinning the minigun once they get there. The significant decrease in movement speed also makes backstabs as difficult as possible, and the spy-checking is trivial. However, the Heavy is one of the best disguises for stealing an ÜberCharge from a foolish enemy Medic.
Engineer	Medium-High	The most useful disguise for disrupting enemy sentry guns. Engineers move at regular speed and generally run around the enemy base and near sentries. To their disadvantage, running around with the shotgun visible and not being seen building may raise suspicion. There is also the chance that they may come into contact with the Engineer that shares their name while attempting to sap their sentries. This disguise is possibly the best for disrupting sentries guarded by multiple Engineers, as it makes the Spy a less conspicuous target if he starts wreaking havoc with his sappers, and enemies won't know which engineer is the spy.
Medic	Low	"Can you fix him, doctor?" A situational disguise. Enemies often have their backs to Medics, and aren't surprised if you run behind them. Spies disguised as Medics move slightly slower than a real Medic's speed, but that won't usually give you away. However, wielding the syringe gun and not healing nearby teammates (which a Medic should be doing near constantly) instantly arouses suspicion from observant enemy players. Your biggest giveaway though is the lack of an Ubercharge meter.
Sniper	Medium-High	Another of the more useful disguises. Spies disguised as Snipers move at regular speed, are expected to have the sniper rifle equipped and are expected to remain behind enemy lines, though they may draw suspicion running around the enemy base rather than remaining in a sniper spot and being unable to raise the scope to their eye. The Sniper disguise is useful for Spies making their way to enemy Sniper spots with less chance of being spy-checked, as well as sneaking up on the Snipers themselves.
Spy	Medium	Another situational disguise. While Spies disguised as Spies receive no penalty to movement speed, they will look suspicious running around the enemy base while appearing to be undisguised. However, it fools enemies at range and gives an excuse to be running from the enemy calling for a Medic or to be mixed with opposing players. Unfortunately, many players instinctively shoot any spy on sight, including friendly ones.
Friendly	Medium	Useful in certain situations. During setup time, Spies disguised as other classes on their own team can get an idea of the class distribution on the opposing side without alerting the enemy team to the fact that there is a Spy, while at the same time misleading the enemy. Likewise, it can be used to infiltrate the enemy base and be spotted without alerting the enemy to the presence of a Spy and giving them false info about the team's composition while saving charge on the cloak. Disguising as a stronger class may also allow fear tactics, for example: disguising as a Pyro to scare off enemy Spies. Spies may sometimes be able to draw enemy fire away from their teammates by disguising as a more important or vulnerable class, such as a Medic or Heavy.

Additional Articles

See Spy strategy for information on how to infiltrate enemy territory effectively using disguises