Well, temporarily, 9mm. This is largely influenced by my chief (big-city PD) authorizing 9mm as an alternative duty pistol cartridge in September 2015. The .40, when fired in my high-bore-axis P229 duty pistol, had become painful to shoot. (Getting old is not fun, and the big-bore Magnums I had fired in the Eighties are probably what caused most of the problem.) I switched to 9mm, for the lesser per-shot recoil, and to Glock for the lower bore axis and the potential additional cushioning effect of the flexing polymer frame.
Notably, 9mm is becoming our standard primary duty pistol cartridge this year. A cadet class which will graduate later this year will have to use only 9mm duty pistols, unless a future chief changes the policy again. (Present officers, at least at this point in time, will not have to switch to 9mm. We buy our own duty firearms, within policy guidelines, and previous changes have generally allowed for existing weapons to be "grandfathered."
9mm is not, however, safe upon its throne. It is simply convenient to carry the same cartridge/platform on and off the clock, and I am likely to retire later this year, or early next year, or perhaps move to a non-patrol assignment. My second-most-carried cartridge is .45 ACP, presently in full-sized all-steel 1911 pistols. .45 ACP, fired from all-steel low-bore-axis 1911 pistols, is not painful to shoot, at least at this point in time. Without the five/six-nights-a-week duty pistol factor favoring the G19/9mm, I might revert to carrying 1911 pistols most of the time, and both of mine are chambered for .45 ACP.
Ii is also quite possible I will largely revert to revolvers after retirement, and none of mine are 9mm revolvers. Candidates include .38 Special, .357 Magnum, and .45 Colt.