As has been said many times before, caliber's not the issue, shot placement is
I use a .177 Diana 52 (broken at the moment ), a .20 HW90 and a .22 King Ratty. I've never had any problems with any caliber so long as I do my bit well but if I had to choose I'd go for the smaller calibers with a sub 12ft-lb gun. The Diana's my tool of choice (when it's not broken ) but I've been very pleased with the results I've been getting with .20.
A lot of folks call it a compromise, but it's not. It's just different to the other two. I don't count .25, it's rubbish. Unless you're shooting rats