What is the most powerful bb rifle fps wise?

First of all BB rifles are not very powerful compared to pellet rifles.

Second, There is a big difference between "POWER" and "FPS"

BB rifles usually top out around 800 FPS Like the Crosman MK-177. There are others that can reach this speed.

Considering that most all BB's weigh the same, then the knock down power is about the same.

Now consider an Air rifle (Air Force Condor) that fires a lightweight pellet. It can reach speeds of over 1,300 FPS. This doesn't make it powerful, just fast. The same rifle firing a much heaver pellet (Pellets come in different weights) will fly slower but will have more knock down power when it hits the target.

Think of it this way. Your on a motorcycle and you hit a fly coming at you at 30 MPH. It may sting, but a bumble bee flying at 20 MPH is really going to hurt. So don't confuse Power and FPS