Define useful here, as well as location. I ask this because medicine doesn't have a linear progression in any way, shape or form. Because while we wouldn't trust doctor from the 12th century, that doesn't mean they didn't help (I'm not saying the help was amazing, but still useful), even if that help involved leaches (which we still use today!). Medicine before the 19th century was very much a guessing game, but a lot of stuff actually worked, even if we didn't understand. Like putting honey or garlic on a wound. Both of those will help prevent infection, not that they knew that.

As for location, I ask because a Muslim doctor in the middle east, in the 11th century is going to be working with very different, much cleaner tools then say, in England. Even things like a doctor who is working at a hospital in London in 1912 is going to be a lot more useful in curing illness then a English doctor in the country side who still believes in miasma theory. Sorry for the questions, I'll try to answer once I get a better idea of what you're looking for.