I think lightsabers win, hands down, in that they are ubiquitously known and also never mistaken for anything else but themselves -- if they show up, be it another
movie or
comedy shows, you know it's an allusion to the iconic ones (whereas if a series has a sword, you might need it clarified that it is Excalibur, etc.).
Otherwise to add to the list, certainly Wonder Woman's lasso, Ash's chainsaw, Poseidon's trident should also count. Others, it depends...
Various figures like Robin Hood, William Tell, and Elmer Fudd are iconically depicted with a specific type of weapon (bow, crossbow, and hunting rifle, respectively), but it's not always clear that they are using the same specific weapon in deferent scenes (and the weapon generically as a type maybe isn't as iconic as something like a lightsaber or tommy gun), so I don't know if it is the weapon counts as iconic so much as the character having one.
Some items aren't necessarily weapons. I count Ash's chainsaw because it is only ever used for violence (by that logic, Bugs Bunny's anvils should count, although see point above about not an individual item). However, what about Potterverse wands (mostly tools, but with capacity to harm and regardless used in combat-like situations repeatedly)? I include Poseidon's trident even though tridents are realistically more fishing tools than weapons, but in the Roman era when he (well, Neptune I guess) was an actual worshipped god, tridents were used in gladiatorial matches. But what about the grim reaper's scythe or devil's pitchfork? Both tools, not weapons, and not actually depicted as being used as weapons.
Others have mentioned the Death Star and A-10 Warthogs. To me, those would be combat vehicles/weapon delivery systems, although their main guns are in fact pretty iconic.
They've already been listed, but Sting, Excalibur, Stormbringer, and lightsabers all belong on the list.
Oh yes, his 'Feyd-Rautha in a metal speedo' depiction in the Lynch version of Dune certainly was a dagger to the eyes. Oh wait, you mean something else...