Randomly commenting on some names that were dropped :
Weezer : It's really Buddy Holly. Darkhorse candidate amongst die hards fans could be Across The Sea.
Iron Maiden : Number of the Beast, followed closely by Run To The Hills. The ultimate fan favorite is probably Hallowed Be Thy Name
Pearl Jam : In the mainstream, Jeremy. I don't think there's any dispute here, and somewhat thanks to a great video. As far as I'm really concerned (and that's based on the few friend of mines who are huge Pearl Jam fans, while I mostly just like them, their most iconic song would be Yellow Ledbetter, a last-minute cut from Ten who ended up on their B-Side compilation). And frankly, I tend to agree with them.
Soundgarden : Again, amongst the general public, it's Black Hole Sun. Amongst fans (and known songs), I'd say Jesus Christ Pose. If you stretch it to more obscure songs (read, non-singles) : 4th of July.
Elton John : Can you feel the love tonight.
STP : Plush (this is a no brainer to me), and I don't think the other ones who have a claim would be on Core (Sour Girl and Interstate Love Song).
Incubus : NONE. Too big of a stylistic shift to have an iconic song as far as I'm concerned.