There is no objectivity. Car liveries are a form of art and just like all other art, it is purely subjective.
Iconic liveries don't necessarily have to be good (although that is a nice bonus). They're iconic because any race fan should be able to recognize them right away. Those liveries easily identify the team or sponsor running them. It also doesn't hurt if the livery has a winning history.
My preference is simple liveries with nice colour combinations. And judging by the "Worst Livery" thread, I think most people tend to agree. Most of the liveries in that thread are either too busy, or have horrible colour combinations, or both. But again, it's subjective. There were quite a few liveries in that thread that I liked, while there's a couple here that I think belong in that thread.