For practical autoloaders (not plinkers / target guns), I will normally only consider .45 ACP. The problems associated with 9mm and .40 mean that there is no real question, in my opinion. A good .45 that's well-matched to its ammo will almost always match or outperform both 9mm and .40. I also tend to prefer Ruger pistols due to their reliability and durability, but would also consider Springfield. Glocks are a bit too vulnerable to ammo problems, in my opinion.
I DID recently make an exception, however, for a number of reasons. I purchased a Romanian Tokarev 7.62x25 pistol. One, it's still a very powerful round, with recoil close to that of a .45. Two, ammo is dirt cheap, allowing me to train without burning my wallet. Three, it's accurate, and snaps back to point of aim after shooting for rapid followup shots. Four, the Wolf hollowpoints reliably expand at least 50%, and the FMJ rounds will go through a Level II vest and mushroom to .46, and still get optimal penetration (.45 ACP rounds frequently do not expand reliably, even with hollowpoints). I got the pistol and 1224 rounds for just over $400. This pistol and caliber created a perfect storm of value and effectiveness, coupled with reliability and accuracy.
For revolvers, .357 or .38. That gives me a universal ammo type, and lets me use the .357 rounds in the large frame revolvers. This is more a rule of thumb, however, as these are immediate home defense guns and thus a large ammo stockpile is not necessary; adding a new caliber will not add a significant ammo expense. As much as I'd like a Blackhawk in .44 Magnum, I'd much rather save my money and get a Super Redhawk in .454 Casull. That will give me similar convertability as the .357 / .38, allowing me to also shoot .45LC, while also letting me shoot a round that's 50% more powerful than .44 Magnum. Same concept, just a lot bigger.
For rifles, I seem to have settled on .223 and 7.62x54R. I'd like a .308, but can't afford to add a new rifle and ammo type right now.
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