I 100% finished the game this past Saturday. And I'm also 100% done with the guide: http://www.ign.com/wikis/yoshis-new-island
This is probably the hardest stage to 100% at once in the game:
Yoshi's New Island Collectibles Guide - World 2-S: All Aboard the Terrain Train
This is probably the hardest stage to 100% at once in the game:
Yoshi's New Island Collectibles Guide - World 2-S: All Aboard the Terrain Train