Generally this question pertains to who makes the most powerful 17, 22, 25. A good one to buy is the Trail. It gets good reviews, is good enough. And it has a lot of aftermarket somewhat like the 870, 700, 10/22, or 1911.
However, if you are asking who makes the most powerful conventional gun, then you might want to rethink your approach. Springers, and even PCPs are rarely simultaneously the best, and the most powerful. One reason is they know they will catch the punters after their first gun by making a cheap gun with a big, usually fraudulent number. Many of these guns are OK, many others are unusable, but putting a pellet on target with effect is actually a pretty complex series of compromises. You have ease of charging, trigger, recoil, velocity, accuracy, noise, cost, vibration, scope breaking, etc...
Overall shootability is the main issue. In the UK it works out nicely, because most quarry is small as with the pigeons and rabbits (a small creature in the UK). Here and in the US, there are much tougher target: Jacks, groundhog, hog, raccoons, etc... So one does often need a fair amount of power. But if you run to smaller animals, or targets, then scaling back on power only is a good idea.