For truely expensive, look at the Gary Barnes big bore .45 pre-charged custom rifles that can be used for big game (deer). Over $4K.
However, I believe you are talking more in the realm of affordable.
My vote for best spring powered guns would be Air Arms of Great Britain, specifically the TX200 series underlever cocking models. Incredibly silky smooth with Walther barrels. Not the highest powered (a common error of airgun newbies), but in the 925+ FPS range with .177 Crosman Premier Lite pellets, which gives 16 FP of energy. The accuracy of a good airgun with pellets it likes will amaze you. For more power, the Air Arms Pro Elite is a break barreled rifle. The TX200MkIII and the Pro Elite have integrally moderated barrels to diminish the noise.
For smooth shooting, the Theoben gas spring models are wonderful, but a bit pricy. Beeman brings in the Crow Magnum III and the SLR98 repeater, but the prices near the grand mark. I found a Crow Magnum in .22 at a gunshow for real cheap, but it was too hard to cock at over 60 pounds. It would however drive a pellet completely through a 5/8 inch piece of plywood. I traded it for another airgun and am more happy and made a nice profit.
The SLR98 is less powerful, but is a 7 shot repeater with an Anschutz barrel. I have one and would never part with it. One hole groups are routine. Expensive yes, but such a well built, attractive gun. The stock is African Hyedua wood. Theobens are essentially hand made.
If you can shoot a spring gun well, with its dual action recoil, you can shoot most anything. Pre-charged Pneumatic or PCP rifles like the Theoben Rapid 12 above are a whole different matter and offer high power (in the USA) with NO RECOIL like you get from a spring gun.
I find airgunning to be even more fun than firearms since I can shoot in the basement even with a "Magnum" airgun.