I almost always get it for the purpose I'm shooting. I had a .45 super for bowling pin matches, a 6mmAr turbo because I want to shoot an AR at a 1000. .22 for smallbore matches, and .223 and 30/06 for High Power.
Guns are like golf clubs. Each one fills a different need. So having just one caliber is almost useless unless your always shooting at the same thing from the same distance.
.22 cause it is cheap and plentiful (well it should be)
5.56/.223 cause you need to have one.
9mm great pistol round and works well as a rifle round also
.357 great stopping power and great hunting round in my lever action rifle
12 gauge well I've always had a 12 gauge
.40 easier to get than 9mm and just a few bucks more and I think you get the most bang for your buck
12ga to me this is the ultimate shtf weapon I can pop you with a slug @100yds and up close 00 buck equalizes multi threats with one pull. Not to mention you can turn birdshot into a nasty SD round with the cut shell method
.223/556 fun was plentiful will be laying all over the battle field if shtf
357/38 devastating round/cheap to plink with
.22lr cheap to shoot cheap to stock up and effective enough as last resort
.22 rimfire - first "real" gun, inherited from my father. Bought a couple others since, and even had 2 that were given to me by people not into guns who know that I am
20 ga single shot - first shotgun, used for squirrel and rabbit when I was a teen
12 ga - acquired as I got more into hunting, now have several sepcifically set up for deer, waterfowl, turkey + pheasant
7mm rem mag - first high power rifle - Ruger M77 high school graduation gift from parents
.270 win, .300 win mag, .338 win mag, .30-06, .243, .223, .300 ultra mag, various calibers collected over the years for different reasons. Some I actually hunt with, some for no particular reason other than I wanted one.
.357 mag - first handgun
collected a few other handgun calibers over the years, but my favorite and EDC is .45 acp
I have arthritis in my elbows and was concerned about recoil. Talked to several folks and decided to start out with 9mm to see how my elbows would do. Turns out it wasn't an issue. Decided to try .40 a try and really liked it. Thinking about .45 after all the hype dies down and prices come back to earth.