Ubtil recently I would have said the boomerang. It was a very iconic item from the first game to the latest and was typically a secondary items in almost everyone's inventory. Not to mention the strangeness of having a medieval character toss a boomerang around, it only really occurred in Zelda. However it has had little to no use in the latest games and now a lot of new fans might not know much of it anymore. It has fallen almost totally out of use.
After that I would say the bow since it is the item most often seen in promotional artwork outside of the typical sword/shield. Plus it is always present in every game in some form no matter what. There are way too many situations that require it but it is the most common mainstay among the medium/long range items. Everything else is only around for one or two games like the beetle or slingshot or constantly changes forms like the hookshot and longshot. It also has the history behind it being an items from the first game as well.
But if I could really choose something from Link that is iconic, I would pick his hat. The hat has been present in each and every last appearance of Link and is often used to represent Link in other characters. Often a green floppy hat is enough to represent Link or even the Zelda series.
Generally you see a green hat and a sword and instantly Zelda comes to mind. Regardless of who is wearing it, you think "oh a little Link" which means that the green hat that easily represents Link is possibly the most iconic items on Link at any time.