I’m just digging into a terrific site posted by William Mackesy. In fact, William’s done about 40% of his top 100 already.
I like it so much that I’m including a dedicated link in the right hand navigation under RECOMMENDED SITES.
Walkopedia – World’s 100 Best Walks
One quibble …
#2 are the “Inca Trails” of Peru: Inca Trail, Silque Valley, Mollepata/Salkantay, …
Normally I discount any list including the Inca Trail. It’s much maligned by serious hikers. That choice is marked “* provisional”, however. Having been there twice, I’ll recommend to William that he downrate.
Choquequirao is mentioned, however. Choquequirao to Machu Picchu is the “best” hike to Machu Picchu, I reckon.
besthike.com 
I’m conflicted on the Inca Trail – having done it, I totally agree it’s by no means even the best hike in the area, let alone the world, from a ‘serious hiking’ perspective – and yet I don’t think I’d quibble with putting it in the list, as I’m not sure there are many other hikes in the world that have such historical or cultural importance, with all the ruins along the way that no-one else ever gets to see.