Depending howe literal you interpret the question, at the present time, the most sought after IWC is the 55mm Heritage Pilot. A limited run of 100 snapped up within days of its announcement which will ensure a lively secondary market. From my point of view, that would watch would not make my list of most desirable. There are many pre-2000 watches that would easily populate the list, particularly if they are in good condition with boxes and papers. They also tend to hold their values very well. I saw a Porsche Design Ocean BUND last week with box, papers, tools and the titanium bracelet still in its plastic packaging having never been attached to the case. That watch would make the list and, sadly, it was reflected in the price.
Maybe, if the watch sells today below the original inflation adjusted retail price, it cannot be considered for inclusion in the list.