So many things to consider. Cost, shot count, weight, loudness.
I own a 9MM Corsair, .495 Quackenbush, .45 Gargoyl, and a .58 Pistol. I love them all.
The bigger and more powerful they get, the bigger my smile.
Personally, I feel a big bore starts at .45 caliber. That is when shooting things really get interesting. Shooting water filled pumpkins, tin cans, mud, and just long range targets is so fun. I also like to feel little recoil and hear a big noise when I shoot.
My favorite gun right now is my .58 Caliber pistol. Shot count of one, really loud and really kicks, and really shoots straight. I bring a small paintball bottle in the woods to refill and I get 7 fills.
My recommendation for the best gun to buy if cost isn't an option is the extreme .45. probably the most power too.
The Texan wins the vote for commercially available best all around gun, I just don't like the "line of sight/line of fire" distance.
The Evanix Rex is by far the lightest but not super powerful. (New models might come out soon)
Texan SS might be the quiet choice I haven't seen one yet.
Umarex Hammer isn't available yet but might be a very cool gun to consider.
What ever choice you make, be prepared to go through lots of air and lots of lead. Plan on casting your own. You can still get 50 pounds of lead delivered to your door for cheap on e-bay.
Have fun with your choice, and let us know what you decide. John