For me it's the .300 Savage. The round is still manufactured, but can be hard to find on dealer's shelves. It's been a lot of years since a rifle was manufactured in that caliber, which is why Savage 99s are constantly going up in value. It's in the .308 class and has always been a reliable killer on critters up through elk at reasonable distances.
A couple years back my eldest son (now 38) showed up at a Texas hunt and surprised all his youngblood buddies with presumably hotter rounds by using his granddad's Model 99 to take a five-pointer.
Grub 348: FWIW, a family friend who manages a gun shop in the area reports that lots and lots of Mexican-American guys come into the store looking to buy .348 ammo, presumably for hunts south of the border, where I gather it is a pretty popular round.
Since neither the Winchester 71 nor the later Browning lever action have been made or chambered in that round for quite awhile, I have to guess there are also a lot of real old-timer conversions kicking around south of the border. Something originally chambered for .405 probably would do to start, since the .348 is based on the .405 case necked down.
Bill