If we look at the winners of each the Cobbled and Ardennes Classics there is not a lot of difference. If anything, up until 1990 to avoid clinic issues, Amstel, Liège & Flèche have hardly any surprise winners. By contrast Roubaix has several, anyone remember Dirk Demol winning anything else? How about Jean Marie Wampers?
But all this really proves is that arguing which is harder, better or more worthy is idiotic. You're all right and wrong at the same time. Neither is better, harder etc, they're different and that is what makes them all great.
in my opinion, and it is just my opinion, Flanders & Roubaix are the races I anticipate the most. I pray for rain in Belgium & Northern France in April every year, FFS! But the Ardennes Double is just as special an achievement.