Make me pick one? No fair. Rush gave five. I will also, in no particular order.

Grand Canyon rim to rim with some off-corridor trails too.
The northern section of the Kungsleden going south from Abisko a few hundred miles north of the Arctic Circle in Sweden.
A long distance AT hike. A thru hike is not necessary for me. But something long enough to get me in shape would be nice. Section hiking sucks.
Summit Kilimanjaro using the Lemosho trail head around the north side. You
Here: https://goo.gl/maps/LWjFLxHT6872 You will have to zoom out to see where. Here's a hint - it's called "The Roof of the World".