In my naive and biased opinion, I'd be inclined to say the White Mountains of New Hampshire, because I've heard the gradings in parts of the country outside of the northern northeast, especially outside of the Whites, are much more rational -- though the eastern US is all pretty tough I've heard, probably because its comparatively less rugged ridges allow and challenge trails to climb virtually anywhere. After all, the 500-mile-longer PCT supposedly has only 300,000 feet of elevation change compared to the AT's 475,200 feet, according to the sources cited on wikipedia.
So, I'd vote for the Whites. Not really talking individual trails, but regions or long-distance trails.