Psylocke's (Betsy's) iconic suit is her ninja bimbo suit with the bands. Her not-iconic suit is her purple armor and hood. And now, Captain Britain. There was also something she briefly wore right when she joined the X-Men (ribbons for sleeves?).
Dazzler has two fairly iconic suits: disco silver with or without skates and her blue X-Men suit. Everything else (like 8 or so different suits now) is not-iconic.
Aurora's most iconic is probably still the black & white. That gold & white one isn't very iconic.
Nightcrawler's classic suit with the flared shoulders is really iconic. He's changed it a few times, but it never looks right. Even though I liked his change in the 90s, with the buzzcut and goatee.
Emma Frost is most iconic in leather lingerie. All of her remotely normal attire is un-iconic.
Sunfire's "rising sun" suit is really iconic. The AOA style suit is second. Anything else isn't.
Rogue went through several different outfits in the 80s, but two have become pretty iconic: green hood with white strips & green and gold with or without jacket.
Other X-Men have iconic accessories, like Storm's tiara, Havok's headpiece, Gambit's trenchcoat (no one cares what else he wears, but the coat is a must), Magik has the headdress thingy that's recently become pretty iconic. They change every so often, but it never sticks.
Storm also has iconic hairstyles: either a big, flowing, cloud-like mane, or a lightning bolt mohawk. Other styles don't tend to work for her.
In short: all of those Cockrum characters in their original designs are iconic, anything else-- not.