What is your favorite caliber

.20 cal being the ideal PELLET GUN caliber. Wickedly accurate and in the weight pellets currently available doable up to @ 35/40 ft lbs.

20 caliber is really an oddball size. It's not large enough to give you good energy and if you look at almost any Airgun the 20 caliber gives you less performance because the pellet is longer for its diameter than other calibers. it has much more drag than a 22 or 25
 
Currently except for two Springers that are 177 cal And by the way never get used. The rest of my guns are 22 caliber PCP I think it's a great all around target plinking hunting diameter that you can buy slugs for and not break the bank. I do believe my next one will be a 25 and I have some interest in a 30 caliber so I will be experimentin with those
 
My airguns exist for hunting first and foremost so hunting versatility is my number one consideration

Its hard to separate caliber from the gun. .300 is my favorite caliber, but my favorite gun has a .308 barrel that shoots both .308 bullets and .300 pellets well and easily adjusts between 80fpe and 200fpe. That .308 is my all purpose gun, but that’s because its acting as a hybrid of 2 calibers. 

I think .300 can kill everything in the southern US woods with a brain shot and most things with a lung shot. 
 
 My favorite isn't my favorite because of how it flies or what it can do, but rather that it is a lot easier for me to see fly. I like to watch my shots and .25 makes that a lot easier, .30 would be better but uses too much air and are too expensive to for me to plink with, and to be honest, kind of a waste for me. I have recently gotten floaters in my eyes and they are a real pain sometimes. It isn't a matter of which caliber is capable anymore, the small .177 is getting pushed to 40fpe these days, but the pellets that do that are still not common and are a mail order thing only. I believe .22 is the most versatile single caliber now, but that wasn't the question either, so .25 caliber for me. I also like how they handle when loading magazines, just a nice sized piece of lead.
 
Good question , I think if I had to choose one, and only one it would be .25 . Having, .177, .22, 25, .30, and 35. The .25 for me falls right in the middle of perfect, sure the .177 is best for plinking all day and on the cheap, and sure the .30 and 35 make the feathers fly but the .25 is perfect for me all around. Not to tiny to load in mags, bucks the wind great , and not to terribly expensive to shoot and a good compromise between them all.
 
First and foremost for me is .22



I have a 'thing' for .22 pups and mini carbines , with the one exception being a .25 Wildcat MKII Compact that is magical in its consistency and performance.



The bulk of my plinking is at 50 yards, and most pesting shots are 15 to 40 yards so .22 works really well for my purposes. 

My Impact chunks .22 Monsters right at 900 fps and is a back yard death ray.
 
I own .22 and .177. The .22 never gets shot unless I'm far from civilization. I don't need any caliber besides these two. If I need more power I burn powder, but then, there are many places where I live that you can shoot as far as you want. Air guns let me shoot anytime I want in town, but I'm careful to stay un-noticed/not do anything stupid to attract attention. My .177 can take everything I desire or need to take, like skunks, so it's my favorite. I just ordered a Sniper HR in .177 in fact-tempting to get the .22, but I'll let all the slug turmoil settle out for now.
 
My .25 caliber around 45 ft lbs is my favorite with the .25 grain JSB.

It's fairly cheap,powerful and performs decent at long range.

For all out fun my .22 caliber at about 32 ft lbs with 18 grain jsb.Sometimes I like to waste crosman cphp 14.3grains at about 25ft lbs since they are $6 for 500.

The one that brings me the most joy is the. 357 JSB at 156 ft lbs.It really does well on small game.

If I had to have one caliber I like a hard hitting .25 caliber.Something like a 34 grain at 900 feet per second,probly around 60 ft lbs