Mind you, these characters may have -classic- costumes, but it’s not the same thing.
For example, Marvel’s Janet Van Dyne made a habit of not having an iconic costume. She’d show up after a long absence in something completely different and fans wouldn’t bat an eye.
DC’s Black Lightning usually gets a redesign after each hiatus, to no outrage.
But then you have your Superman and Captain America types. You can -tweak- their costumes, but not too much, and even tweaks usually generate fan response on noticeable scale.
So, well known characters, least iconic costumes.