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.
Others have offered up a sighting of roughly 2 inches high at 100 yards as a good sighting scheme. In my own experience I have come to favor a sighting of 3.5 inches high at 100 yards. This allows for the individual to hold dead-on (directly in the middle of the top and bottom) the animal out to roughly 350 yards.
Magnum calibers such as the 7mm Remington and 300 Winchester will extend this slightly. At 400 yards I hold directly on the backbone of the animal. The drop at this range allows the...