.22 as that was the calibre most people used in my developmental years. My pal got a new BSA Mercury in .177 early 80s and we all had .22 so we all shared experience of the two calibres prevalent at the time. Never had a bias but have tended to buy and use .22 for pest control. If you know the capabilities and and limitations of the pellet and what pushes it, combined with being able to consistently hit the humane kill area, those are the critical considerations. Used a .177 Theoben for rabbit control on a farm a while back and that was exceptional. Have never had the opportunity to learn and try .20 or .25.