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.
Here is the scenario... you sit in your 2 piece climbing stand and your foot inadvertently bumps the lower section while there is no weight on it - and it slides down the tree, out of reach!
OK, you are safe - you have a string tied from the upper section that you are sitting on and the foot portion that slid 4-5 feet down the tree. All you have to do it pull the other section up, reposition it and you are back in business, correct?
Have you ever tested this theory?
I have -...