40 S&W. I shoot a lot, so I have to keep costs down, and I also load my own. 40 can be used with light loads and bullets for minor caliber shooting like IDPA, USPSA Production, and the Steel Challenge. It can also easily make USPSA Major for use in Limited, Limited 10, and even Open, but is not very popular with the "Space Gun" crowd as .355 pretty much dominates this division.
Due to its adoption by LE it has become extremely popular with civilians. The end result is that 40 cases are available for free at your local range. I can't remember the last time I bought a .40 case, probably just after 40 hit the market, 20 years ago.
Another advantage is that the 40 uses lighter bullets than the 45 and lead has become very expensive. I do most of my 40 shooting with 155 gr lead SWCs. Using this bullet, I can duplicate the performance of a 200 gr 45 bullet for less $. The 40 round uses less lead, less powder, and the brass is free.
As you can see, my love of the 40 is based solely on its use in practice and competition. This caliber allows me to shoot more fore less $. In the real world I would most likely resort to a .45.