The grizzly bear exerts the same sort of fascination as the great white shark or the graboids from Tremors. Which is to say, it's hard not to imagine them eating you. The same might go for the tyrannosaurus rex. But unlike those three, the grizzly bear lives in your land, actually exists, and is not extinct.
That settles it, then. Grizzly bears have them all beat as objects of terror. Plus they're pretty cute.
Assuming that you don't want to be eaten by them (arguably as opposed to Timothy Treadwell, subject of Werner Herzog's documentary Grizzly Man) here are a few rules which will dramatically decrease your likelihood of being killed during a grizzly attack:
If a bear charges, stand your ground. In most encounters, the bear will stop short at the last moment in what is known as a bluff-charge. Don't make eye contact, which bears take as aggressive. Likewise, and for the same reason, don't crouch. If possible, back away slowly while making soft, non-threatening sounds. If you have something you can toss aside to create a distraction, do it -- unless, that is, that thing is food. Then definitely don't throw it. Don't even run unless you've got a good deal of distance between you and the animal, because grizzlies can cover 40 feet in an instant. It follows that climbing a tree isn't a good option unless the bear charges from quite a distance.
If you do not have a weapon (and sometimes even if you do), then the best option is generally to do everything you can to make it seem as if you're not a threat. Get onto the ground, face down, and put your hands behind your head to protect your head and neck. Being on your stomach will help to protect your vitals, and if the bear moves you, get back onto your stomach, and resume laying still. It happens that the old cliche is true: play dead. Bears tend to lose interest in attacking when the threat is perceived to be gone.
What if you do have a gun? Even then, it might be best not to take the shot. Even expert marksmen are likely to be ruffled at the sight of a charging bear, meaning there's a good chance of missing or landing a non-fatal shot. And a bluff-charge easily becomes a real, fatal charge when a bear is injured or enraged. But if you do choose to shoot, aim for the bear's head and start thinking about how you're going to prove to a skeptical park ranger how this was in self-defense, provided you survive that long.
If you have bear spray, that's a better option in general - park rangers have relied on it for decades. Spray the whole damn right can into the bear's eyes. Hopefully, the pain is enough to deter the bear.
Finally, if you believe in a God, pray to him. Or her!