What is the highest quality pellet gun?

When you say "Gun" do you mean Pistol or both Pistol and Rifle?

The highest quality pellet guns are Olympic class Pre-Charged Pneumatic match pistols. (PCP's). An example would be a Feinwerkbau P44 Match Air Pistol. The cost for this pistol is $1765.95. The very best Match rifle is the Feinwerkbau 800 PCP Air Rifle at a cost of $3245.95. The support system to charge the rifle or the pistol will cost anywhere between $300 to $1,000.

However is you want something a little less but Excellent, then there is a very large selection of pellet guns available that will cost you anywhere between $300 to $600.