Define Iconic.
Really it'll come down to familiarity and legend and, come to think of it, nostalgia/longevity. Familiarity with low-level spells as most games fizzle at level 7 or earlier. Legend because they're the "obvious" ones that newbs like me would go for. Longevity/Nostalgia because they've been around so long.
Familiarity: magic missile, cure light wounds, fireball, dimension door, mage hand, and Detect Evil or Magic (more than other Detects).
Legend: Wish, disjunction, Magnificent Mansion, Shape Change, Polymorph any Object. Teleport and Fly, because they change the game.
Nostalgia: All of the above. If you can remember the name through different editions, it's got Longevity/Nostalgia value. Also, if you know what it is, PLUS you'd never really use it. I'd put the Law/Chaos/Evil/Good spells here. Magic Mouth and Magic Jar, too. Who really uses "detect Law"? I mean, really? Or Magic Mouth? Sure, sometimes, but you wouldn't see a Sorcerer use it outside a scroll or wand.