Hand's down, the High Sierra Trail in Sequoia National Park. Search it on Wikipedia. Starts in Giant Forest (Crescent Meadows Trailhead) which has largest trees in the world, ends at Mt. Whitney, tallest mountain in contiguous 48 USA states. " From Crescent Meadow to Whitney Portal the trail is 72.2 miles (116.2 km) long and is often hiked in 6 to 7 days or more. There is a natural hot springs in the middle. It is occasionally performed in even less time by experts. Work began on the High Sierra Trail in 1928 and it was the first Sierra trail built solely for recreational purposes."
PS. Best way to do this round trip, is to return from Whitney Portal via Horseshoe Meadows Trailhead after resupplying at Lone Pine, CA, and you can hike due West via Golden Trout Wilderness Area to Kern Canyon Ranger Station via a well maintained trail and then north 9 miles to the High Sierra Trail, then back to Crescent Meadows. That is about 130 miles, via that "lasso" route. Mt. Whitney Shuttle Service can help at Lone Pine, CA. And a shuttle bus works from Visalia, CA to Lodgepole, CA then to Crescent Meadows Trailhead. Round trip is easiest way to deal with transportation. Doing a one way trip, requires you to do an "open jaw" flight to Fresno, CA to start, and from Reno, CA at end, using buses to get to/from trailhead.