I definitely think it's the Triforce, cause that appears in every game in some way, but not everything else does.
When you see a sword, or a shield, Zelda might not be the first thing that comes to mind, but the Triforce is definitely gonna make everybody think Zelda<3
The most iconic thing in the Legend of Zelda is no doubt the Triforce. The symbol has appeared in every mainstream Zelda game if memory serves correct in some way, shape, or form and further serves to tie together the three main franchise characters-Link Zelda, and Ganondorf/Ganon. The Triforce is an element you can't remove from the series otherwise Zelda is no longer Zelda and the world goes down in ruin.