Only been on 3 so not much experience backpacking out of the US....
Kerry Way in Ireland and
Della Falls on Vancouver Island in BC
North Coast Trail on Vancouver Island BC
Kerry Way was in the off season with my daughter. Great people, great scenery, great history and great hiking partner.
Della Falls was much shorter but more of an adventure, I kayaked to the trailhead and did it in the summer.
North Coast was probably the hardest, had the most people and the most wildlife. Was wet and muddy everytime and saw/heard several crazy stories on that trail. Done it three times, twice with my daughter and once by myself.
Pick one to do over again it would be the Kerry Way...