What I currently own: .38spl, .357mag, .45LC. I used to own a .41mag, got rid of it (even though I loved the caliber) due to the limits on commercially available ammo and reloading components, yet I find myself drawn to it and will likely own another soon.
As for which revolver calibers are "relevant", I'd say all of them. Some of the less used older historic calibers have historical significance. All of the common calibers have their own unique strengths and limitations, and each of us may make different choices when weighing these strengths and weaknesses.
I do want to note a few in particular:
-.22LR. I don't like .22lr in handguns, I've found the pistol I had boring. However, I do want a SA revolver in .22lr, and it is good for cheap practice.
-.22mag. For those who are recoil sensitive, 8 .22mag rounds in a revolver can make a good self defense choice.
-.327mag. It is on my radar as a great J-frame snub chambering. Power levels close to some of the lower level .357mag offerings, and superior to pretty much any .38+P, with lower recoil levels than .357mag. The ability to download to .32H&R Mag for acceptable power levels (comparable to .38spl) with low recoil. The ability to carry one more round in the small snubs than .38spl. I definitely see one in my future.
-38spl and .357mag. Not much to say, there is a reason it is sold as much as it is.
-.45LC. My favorite big bore. It can be a pussycat with cowboy action loads (in a big N-frame it can recoil lighter than a K-frame .38spl), and in a modern revolver you can have some powerful rounds that rival .44mag in power levels.
.45ACP. The .45ACP entered development in 1904, with the design finalized to what we know now around 1908 or 1909. As we all know, the pistol that defined the cartridge was the 1911, which came out in, well, 1911. By 1917 .45ACP was also chambered in revolvers. Thus, for almost the entire history of this cartridge, it has also been a revolver cartridge. While I personally choose the .45LC (and I may buy a .44spl and another .41mag) for my big bore revolver needs, the .45ACP is indeed a classic revolver chambering.