What is the best weapon for a scout on team fortress 2

Personal choice.

The scout has 4 primary Weapons, 5 Secondary weapons and 8 Melee weapons.

So 160 different combinations.

Most scout loadouts that I have seen are usually:

Primary: Scattergun, or Short Stop
Secondary: Bonk!, Milk or Pistol (Lugermorph)
Melee: Sandman or Atomiser