Tough call, but my favorite is 9 mm, due to the accuracy and overall reliability of my 9's, the comfort of shooting a lot and the lower cost of ammo. But I like the .45, too and carry a .45 because I think it is a better self defense round and my short range accuracy is good enough.
Like Allie, I really like .38 Special (and .357 Magnum). These cartridges are both easy to load and extremely versatile. Unfortunately, they don't work well in autos. Now if the question was which automatic pistol cartridge I like best, the answer would be .45 ACP.
45acp is my all around favorite. I don't really know why though.
I am attracted to 9mm too though because the ammo is cheap, and there seems to be a wider variation in offerings in that caliber. Though since the 94 ban, that particular gap between 9mm and 45 has closed significantly.
Well so far in autos, all I use is the 9mm for the cheap ammo and gentle recoil but I'm curious as Hell about the .40 and .45 -- too nervous about firing a gun in that caliber though -- Glock or Beretta. I do use a .38 in my .357 Magnum and love it....
I love the .45 acp, but the .40 S&W takes a close second for me. To me both rounds are powerful, the .45 seems to be alittle more accurate. Although the platform most .45's are shot from (1911) could be a lot of the reason for this. Pistols for both calibers are available in a wide variety of models, but at this point the edge will go to .45 in this catagory as well. As far as availabilty of ammo and a wide assortment of ammo, .45 hands down. The .40 is still too new and I doubt it will ever catch on like the .45 acp. But .40 to me is far better than 9mm. Yes I do own a 9mm and shoot it quite often, but not nearly as much as my Kimber Custom or Beretta 96.