What is the most high power air gun?

It's not just one air gun or rifles.

it's a class of air rifles/guns known as PCP (PreCharged Pneumatic) air guns/rifles that require a Scuba tank, special hand pump or electric pump to charge them to 2000 to 3000 PSI. As an example of the power a normal 12 gram C02 holds 800 PSI these PCP's hold much more and are capable of projecting a MUCH larger pellet. The largest of these is the 50 caliber Dragon Claw by Sam Yang. It is capable of bringing down a deer or Boar.

See the link below.