Well, both of them are wild animals, so both are aggressive, but probably a Grizzly bear. Black bears are more curious than anything, and will only attack when threatened or if you look like prey. Grizzly bears will attack on the same circumstances, but fed bears [ bears who have been fed by humans ] can become agitated if you don't feed them, causing them to attack. Both can kill, but I'd say that Grizzly bears will dish out more damage, more dependent on their size, anyways.
I would have to say that a grizzly is more aggressive.
A grizzly, of course. They are predatory creatures who would just as soon eat you as look at you. Black bears (generally speaking) are peaceable unless you annoy them, and are usually shy and avoid human contact.