If you mean one handgun with no limit on accessories, I'd have to go with a .40 caliber 226 with conversion parts for .357 SIG, 9mm, and .22. Of course you'd need adjustable sights for different loads, as well as multiple magazines for each caliber. It would be damn expensive for a single weapon.
Maybe a P250 with all the different sizes and caliber options. It's not the best shooting weapon out there, but the variety offered for a single serial number is crazy. You can get 9mm, .40, .357 Sig, and .45 in subcompact, compact, and full size only using one drop in frame piece.
Would have to depend on the situation. If the defecation has hit the rotary oscillator, I wouldn't need to conceal the 226. If concealment was necessary, I'd have to go with the 250.