My favorite cartridge is 7.62x39... for all the reasons it was designed, for all the potential that was realized since, and the potential it yet has.
From supersonic to subsonic, from 110gr to 150gr projectiles (supersonic) from 180gr to 220gr projectiles (subsonic) using bullets from .308" to .311" in diameter, while doing it in such a compact package, it's versatile to say the least. It equals or betters the 30-30 Win, 300 AAC BLK and Wilson 300 HAM'R, plus an assortment of also-rans that tried but didn't catch on..
My favorite rifle (carbine in this case) is an early model Ruger Mini-30 with 18" barrel I bought new in 1989. It's an all around one-gun if you could only have one gun. It's a fun plinker with cheap ammo, and a hunter of small to medium game within 250 yards. It's great for self defense, home defense, community defense, with a realistic reach of about 500 yards, for whatever you might need to hit 500 yards away. (not game animals)
It can be dressed and pressed into may roles. I've got magazines for it in 5, 10, 20 and 30 round. Optics include two scout scopes and a couple conventional receiver mount scopes if needed, or I can just grab it and go with irons. It's still dressed in its original wood stock, but I recently picked up a factory black synthetic just to have around.
I've spent more on accessories and ammo for my Mini-30 than any other gun I own. I guess that says it all...
Not far behind as a second favorite is a more recently acquired CZ 527 with 22" barrel, also in 7.62x39.
This pic of my Mini-30 is going back a few years. I need to take a few new ones. It's configured quite a bit differently now.