Spy

The spy is a suave backstabbing rogue that has the ability to insta-kill any class in the game with their knife. Spies are valued in that they can get important picks through the manipulation and deception of the enemy team. The spy arguably has the worst direct combat ability, as it is solely designed to only get one or two kills at a time. As a pick class, the spy will usually target the enemy Demoman, Heavy or Medic and either get the Medic to pop his uber or to drop his uber. The Spy is also capable of incapacitating and destroying the Engineer's buildings.

6v6

Main Article: Spy (6v6)

In 6v6, the Spy is considered an offclass. It is most often run when one team has a disadvantage and they need to try and create an advantage (usually through dropping the enemy Medic's uber).

Highlander

Main Article: Spy (Highlander)

In Highlander, the Spy is often used to pick the enemy Medic, Heavy, Demoman, Sniper, and Engineer. Spies in Highlander must be aware that the enemy team has a full-time Pyro and is spychecking more often, leaving the Spy with less opportunities for picks.

The Spy is also the best scouting class in the game, providing invaluable information for his team. He can find how far along a Medic is to fully charging his ubercharge, where the enemy team is located, and in what direction they are moving towards in the safety provided by his watches.

Primary

Revolver		The revolver deals a base damage of 40, with a clip of 6 rounds. This weapon is a perfect choice for spies that either don't have the time line up head shots, or just feel more comfortable with consistently hitting body shots.
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	Yes
ETF2L HL legal?	Yes

Ambassador		The Ambassador deals a base damage of 34, with a clip of 6 rounds. Most notably, it has perfect accuracy on the first shot, allowing for 102 damage head shots, but has an accuracy fall off on a short cool down that starts after every shot.
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

L'Etranger		The L'Etranger deals a base damage of 32, with a clip of 6 rounds. On every hit, the user gains an additional 15% cloak. This weapon is extremely effective when coupled with the dead ringer due to the fact that it enables the player to regain the cloak needed to feign. This weapon is a necessary component in the Saharan Spy Set.
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Enforcer		The Enforcer deals a base damage of 48, with a clip of 6 rounds. The Enforcer is known for having the highest base damage of all revolvers, with an additional damage boost of 20% when undisguised. This revolver has had a long history of controversy regarding it's balance, specifically in comparison with the Revolver. Similar to the Ambassador, it has -20% firing speed, and cannot get random crits.
ESEA 6s legal?	No
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	No

Secondary

Sapper
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	Yes
ETF2L HL legal?	Yes

Red Tape Recorder
ESEA 6s legal?	Yes
UGC HL legal?	No
ETF2L 6s legal?	No
ETF2L HL legal?	No

PDA2

Invis Watch
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	Yes
ETF2L HL legal?	Yes

Dead Ringer
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Cloak and Dagger
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Melee

Knife
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	Yes
ETF2L HL legal?	Yes

Conniver's Kunai
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Spycicle
ESEA 6s legal?	No
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Big Earner
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Black Rose
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	Yes
ETF2L HL legal?	Yes

Your Eternal Reward
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

Wanga Prick
ESEA 6s legal?	Yes
UGC HL legal?	Yes
ETF2L 6s legal?	No
ETF2L HL legal?	Yes

External Links

DarkNecrid's Comprehensive Pub & Competitive Spy Guide