9mm Parabellum
I've never heard 9mm referred to as that (had to look it up to be sure) but yeah 9mm and 5.56 NATO rounds are great choices as they're available basically anywhere here in the states. So is 30-06, 308, and 22LR.
Personally I've got guns that take 5.56, 30-06, and 22LR. The 22LR are consistently the cheapest ones to shoot and I usually buy them by the box of 500. Among my friend group the only deviation is those that have handguns, they've got guns that take 9mm and 38 special as well.

Yeah the 22LR is for fun for me not for defense. The 5.56 and 30-06 are quite expensive per shot by comparison so they're shot less for fun and more for making sure I'm still skilled with them.