Which caliber would you use for all of the above if you could only have one to chose from?

9MM Luger.....

.40, makes major in USPSA and has good capacity. Minor power factor requires a much better shot for the same score as major. Good punch for CCW. Production class shoots 10 rounds in USPSA and is all scored minor, but limited is high cap. It depends what you want shoot. Different holsters and mag holders too.

Probably a 9mm due to the unmatched accuracy ....

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S&W 1911ES
S&W M&P 9c

.45 1911. Accuracy to shame the 9mm and .40, more wallop on the business end than either, less recoil than most 9s and .40s and is done at lower pressure. When I use a .45, the target stays shot.

IDPA and USPSA have become too diverse for there to be only one answer to this question.

If you want to be truly competitive, better pick the one you shoot best.

If you have a favorite carry gun, it can still fit into USPSA single stack (.45) or Production (9mm) and also IDPA.

Obviously, if your game is USPSA Open, that's a tough one to carry concealed.

I shoot 9mm Minor in IPSC or 45ACP for Single Stack. For IDPA or carry, 45ACP or 357SIG in pistol and 38+P in 357 revolvers. I dislike 40SW and believe it is the firearms and LE blunder of the century. If I had to choose one, it would 45 ACP hands down. Of course, then I would have to reload.

Personally, I used a 9mm for IDPA and USPSA and conceal carry a 9mm as well. The main reason for me was simply the cost of buying factory ammo. Lots of rounds used in USPSA.

A .45acp 1911 or a revolver loaded with .38 Specials serve my needs for both IDPA and CCW.

9mm for carry and IDPA. Quality ammo w/proper placement=good results. For competition, cost savings of 9mm is significant when compared to the others. May not work for you, but works well for me.

If you have to limit yourself to one gun then a 9mm would be my choice. I would opt for the S&W 99, S&W M&P9 or Glock17 or 19. I actually prefer the S&W 99 which has the same ergonomics as the S&W M&P9 has DS/SA trigger and a wonderful albeit unique ambidextrous safety.

In IDPA I shoot S&W99, Sig 229, S&W M&P9 Pro or CZ-75. In USPSA I shoot the S&W M&P 9Pro, Sig 226 and S&W 625. For concelaed carry I use either my S&W 642 or Kahr PM-9. At one time I did carry a Glock 36 in 45 ACP but found it was too difficult to relaibly conveal all the time.

I think a 4" .45 ACP model 625 is the perfect way to go. For IDPA and USPSA competition you get all your brass back in neat little 6 round packages. For CCW you get to use a .45 acp. I could go on and on but I'll just throw that out for your consideration.

Since your question did not take into account ammo cost, I would say that the .40SW is the obvious choice.

This round is entirely adequate for CCW, as evidenced by its use by so many law enforcement agencies.

In IDPA, it will work in both SSP and ESP classes.

For USPSA, it will work in a Production gun (although scored minor), as well as in Limited and Limited 10. Some have set up .40SW guns for use as Open guns also, but they are hardly carry guns.

However, if you only want to shoot USPSA Production as well as IDPA SSP and ESP, then 9mm would be the choice

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NRA-USPSA-IDPA-ICORE-GSSF

I think you should put things into perspective . . . the most important consideration here is CCW. It is your life, where IDPA and USPSA are games. Given, they help you train to better be able to defend your life, but self-defense is at root of all this. Choose a caliber that you are confident will be the best one to defend yourself with first. Then compete to improve your skill with the chosen weapon. Make mine a .45.

A 1911 or a S&W revolver, either in 45ACP.

If I shot a lot of competition, I would want 2 near identical guns.

Say a Govt. Model for Comp, and an Aluminium framed Commander size for CCW, or a 4" ACP revolver for Comp. and a 325 NG for CCW.