Spy
The spy is a suave backstabbing rogue that has the ability to one-shot 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 has the same role: it is often used to pick the enemy Medic, Heavy, Demoman, or other picks most valuable at certain moments (most often the 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.
