Shouldn't Naruto's RasenShuriken be a forbidden Jutsu if it's so powerful? Naruto's RasenShuriken can usually kill average shinobi in one shot. If it's so powerful shouldn't it be forbidden?
It's not typical of strong techniques to be forbidden. Forbidden jutsu are often techniques that either damage the user, require a sacrifice (other than just Chakra), or is generally immoral.

Incidentally, Tsunade did declare Fuuton: Rasenshuriken as a forbidden jutsu after seeing the damage it caused Naruto's arm. Whether or not it's still forbidden after Naruto perfected it and managed to throw it (thus, nullifying the damage to himself).

Other examples of forbidden jutsu:


Izanagi - Because it sacrifices an eye.
Tajuu Kage Bunshin (Massive Shadow Clone) - Because the user might die from chakra loss
Edo Tensei - Because of living sacrifice and immorality.