Pyro. You don't have to know how to play, or various techniques. Just rush into biggest group and incinerate everyone.
Second favorite is Soldier. With sharp aim and Direct Hit, he is invincible. Good for destroying Engineers hard work, and easier to master than Demoman.
Third, Scout. Especially for Intel running and on maps where Force a Nature can toss people off cliffs or into other environmental death.
Least favorite: Demoman. He may have badass swords and axes, but it's hard to use them, and his primary weapons are really hard to master.