If you're looking for the most powerful air rifle that your money can buy, you're going to want a pcp air rifle.  Pcp air rifles use compressed air at 2500-3000 psi to deliver powerful shots and with greater accuracy.  They generally have a built-in tank or a screw-on tank which holds the air.  Depending on the tank size, the first 30 shots or so will be the most powerful shots and with the greatest precision.