In terms of air rifles, Umarex make some of the best and most powerful CO2 air guns. There is always a mix of reliability, shot count and consistency when it comes to how powerful and efficient a CO2 rifle is, but a really good example is the Umarex 850 Air Magnum, which shoots around 11.4 ft/lbs with 88g CO2 capsules, and gets approximately 175 shots per capsule.
CO2 guns are good, depending on what you want to use them for. They are virtually recoilless which means that they are easy to fire and aim with, without a lot of practice, making them perfect for younger shooters. CO2 pistols are often semi-automatic, which means that you can shoot as fast as you can pull the trigger, which is a lot of fun too! In general, CO2 guns tend to be less powerful than other types, so we do recommend other guns for pest control (but there are exceptions)!