One-shot kill depends entirely on your ability to place the bullet properly. You could be hunting with a .458 Magnum and still not get a one-shot kill if you don't do your part.
Assuming you can place the bullet properly, though, any 180 grain bullet from a .30-06 will do the job on a black bear. Personally, I like the Hornady Light Magnums, but Federal ammo with Nosler partition bullets work very well also.
Of course, "one-shot kill" doesn't mean the bear will drop dead on the spot. No cartridge that can be used in a shoulder-fired rifle can guarantee that.
Are you looking for the best cartridge(ie. 30-06, etc.) or the best bullet for your 30-06 that you are using. Any controlled expansion bullet should do the trick. I like the nosler partitions but am loading up a recipe using the barnes triple shock which should work very well.
My very experienced hunting partner uses 220 gr round nose corlockts. Makes an exit hole you can stick your fist in. He has shot a bunch of bear with it. I've killed a few with an 06 and 180 partions, drops them on the spot.
My personal preference is a .358 win leverr rifle with .250 speers. I've shot 17 bear with this rifle and none required tracking, including a really big one I killed in June. (557 lbs and 22" skull squared over 7' 4"
I've seen and killed bears with just about everything and the .35 cals. work exceedingly well as does the 45-70 A magnum is more of a liability than an asset. When hunting black bears.
We all spend our time at the range, making sure our rifle is shooting just right. We get it dialed in, and then we head to the woods, ready to shoot our animal. Well, what happens if the animal comes out where you do not expect it? Will you be able to make the shot?
This scenario has happened to me. My first deer I ever shot, I had to shoot left handed. I had never practiced that way, and paid for it by having my nose broken with the scope. If I had just...