I'd like to say either the original Cooks dogbone cranks or the Grafton canti's as personal favourites of mine - but they were too expensive for most to own and therefore not mainstream enough. For it to be truly iconic, it would need to be in reach of the majority and preferred by most. In this regard the XT Thumbies really are up there as a top candidate - but would add either the Selle Italia Flite Titanium or the SI Turbomatic Titanium as two other potential choices as probably the two best saddles ever made and far better than any Fizik tat that's mass produced today!