I've got one I like to do from time to time that is 3 or maybe 3.5 miles at an average of 8%. It tires me right out, but that's partly because it falls in the middle of a route that is anywhere from 25 to 60-something miles long. (I sometimes drive to a starting point near the climb and make a small loop over the top, then down another canyon and back to the car, but other times I ride all the way to it from home.) It's called
North Ogden Divide.
Here is my latest effort on it:
A better map:
I've done short climbs, like a mile or less, at 12 or maybe 15% gradient. I have no trouble climbing 4 or 5% for many miles. But my threshold for enjoyment with a combination of distance and steep climbing is probably the one mentioned above.