6. 24 (FOX, 2001-2010)
Mystery: Who is the mole in the CTU?
The first season of 24 (2001-2002) was filled with plot twists, action, and death, but nothing was more important than the identity of the mole who is working inside the CTU (Counter Terrorism Unit). There are plenty of red herrings thrown at audiences to keep them wondering who it is, and even if you guess the identity correctly something comes along to temporarily debunk your theory.
Like any great mystery show, when the season drew to a close, the identity of the mole is finally revealed. No spoilers here, but the revelation is wonderful not only for the person's identity but also for how they forever change agent Jack Bauer (Kiefer Sutherland) with their double-crossing actions.