Largest or most powerful?
Largest auto caliber? .45ACP. Sure, .50AE is bigger and more powerful but the guns are too big for most people to reasonably carry or conceal.
Most powerful auto caliber? .45ACP. Sure, 10mm is arguably more powerful, but there are more defensive loadings in .45ACP, there are more concealable guns to choose from, and a good .45ACP loading in the real world probably isn't any worse than 10mm.
Largest revolver CCW caliber? Well, anything larger than .45LC is likely to be in too big a gun to reasonably conceal or carry so I'd have to say .45LC. In addition to the large framed S&W N-frames and similar, .45LC is available in a few more CCW friendly guns as well (Taurus Tracker, Taurus 450), though only available used right now unfortunately. Also, a revolver chambered in .45ACP would obviously qualify as well.
Most powerful revolver caliber? Well, again, the powerhouses in .454, .480. .500, etc. are going to be simply too big for reasonable CCW, also the recoil is such that you won't have reasonable follow-up shot times. Some people may find .44mag to be concealable and controllable. Many people can and do conceal the large S&W N-frames, also you can now get a .44mag Taurus Tracker (basically between a S&W K-frame and L-frame in size) with a 4" barrel. Going down to .357mag is probably more realistic for more people with tons of options in platform (from N-frame to J-frame in size and all in between), in a mid-sized or larger gun it is controllable, and some loadings are more powerful than many .45ACP loadings. Another option is the .41mag, less recoil than the .44mag, more powerful than the .45ACP or .357mag, again many can conceal the large N-frames, and it is available in the 4" Taurus Tracker or the 2 1/2" Taurus 415.