#1 22LR and 45acp would be a tie
#2 44Mag
#3 41 Mag
I shoot a lot of 45ACP but I use that caliber in several competition types(+/- 20,000 a year?) I am not sure but it is a lot. 99.99% are reloaded by me on a 550B. I may shoot a little less 22LR but I do shoot a lot of them(I can shoot in my yard and I go through maybe a brick a week). I love shooting the 44 and 41 mags the most though. They are my 2 favorite cartridges. I have been working more and more lately so my shooting is down some with my own guns.
The rest fall about even but #4 would probably be 9MM. I got a PX4 Storm a few years ago and I have just now started shooting it a lot. It is a blast to plink with. More fun than a 22lr and if you reload a large volume of 9mm all most as cheap. When I get back in the states this summer I hope to really start shooting again more.