The most dangerous bear is the Grizzly Bear.
The Grizzly bear seems to have the most attacks on humans and if you do get into a fight and attack with a Grizzly bear then it can be deadly for you or at least severely injure you.
If you do come up against a Grizzly bear make sure to back away slowly or keep your distance if you're not too close to it.
Don't stare at the grizzly bear as it may see you as a threat.
As long as you keep your distance from the Grizzly bear you should be okay.
Still it's a good idea to keep some bear spray with you just in case you need to use it.
However your best option when possible is to just leave and back away slowly to avoid getting near the bear.