Excluding the sword/boomerang/bow/bombs/instrument that's in virtually very game, what's your favorite item in the series?
I think a good item should
>allow for creative puzzles beyond just use X to progress
>have its puzzle uses fit the game world instead of working on conveniently placed things that interact with it
>be useful outside of puzzles
I think the Pegasus Boots are a good item. They're used in plenty of puzzles, and the puzzles use them in plenty of creative ways: You can use them for the expected use of moving quickly to do things in short time, but you can also use it destroy large segments of grass and to knock into things for effect. Since they let you move at high speed, they're useful pretty much everywhere in the overworld for going fast even after you've solved all the puzzles they're in.
Minish Cap's Gust Jar is another good one. Its suck and blow functions are both very multi-functional. Sucking can not only remove obstacles, but catch certain projectiles, and cause Link to stick to certain surfaces, while blow can not only shoot stuff, but redirect stuff, and propel Link's vessel. It's not super good outside of puzzles but hardly useless.