What is the most powerful .177 pellet pistol?

[Backlighting]

I prefer the spring-piston type.

 

[steamloco76]

The HW-45/Beeman P-1 Magnum is hard to beat @ 600 fps+.

I've had my P-1 .177 for nearly 25 years! Still packs a punch and is ridiculously accurate too.

 

[pownal55]

I believe RWS has a break action pistol with the highest velocity. It may have the same action as the above mentioned Beeman.