I wouldn't say there is ONE iconic campaign for SR.
If pressed, I'd say that Bug City, Arcology Shutdown and Brain Scan had the most impact on the average citizen in the game world, however.
Harlequin is cool and all, but the impact is mostly between two geriatric dandelion eaters and doesn't have much relevance overall.
Harlequin's Back DOES have impact on the world as in "it is saved", but again, no one really gets to know it (and frankly: I hate world-saving plots in SR). But it does satisfy the 'epic' requirement, something that most other SR campaigns don't.
Schockwellen is _really_ cool, and a worthy and even epic finisher for 3rd edition, but not widely available outside Germany, iirc, so there's that.
The MOST iconic campaign (and that doesn't mean it is THE iconic campaign), in my opinion, is Super Tuesday. For obvious reasons