So no one here is leaning towards the Nitro piston or NP2?
Pretty unlikely to find supporters of going this route on here. The reason is largely two-fold:
1) Don't get sucked into the power numbers! Power without accuracy is nothing. A pellet going 1000 fps past the squirrel's head doesn't do any good! All the marketing hype on power and speed is overblown and just that: hype. Higher-powered spring piston guns (and gas/nitro pistons are a type of spring piston) are very hard to shoot accurately and consistently.
2) The difference in quality between the typical Chinese-made guns you find at most big box outlets, and the German springers everyone above is suggesting is HUGE. Unfortunately, so is the price difference. But between spending low-mid $200 range on a "high-end" chinese gun, or $250-$400 on a German-made gun, there is no contest: the German guns offer a much better bank for the buck.
But my first question is how far do you plan on shooting and what kind of pests? For example, if you're looking only to deal with starlings, squirrels and rabbits up to 25 yards, I'd argue that you don't need nearly the power of a Diana 34 or any of those "NP' guns. An HW30 would do just fine and I just about guarantee you'll be able to shoot it with more precision off the bat than any of the higher-power guns. In my backyard, it's all I ever reach for unless a raccoon decides to start knocking over garbage cans.
If you do need the power, I'd also say the Diana/RWS 34 is a good choice. Pretty hard to beat from a bang-for-the-buck perspective.