There are 5 'monuments'. These races are considered 'Classics', no ifs or buts.

Milan-San Remo, Tour of Flanders, Paris- Roubaix, Liege-Bastogne-Liege (all held in spring), and Tour of Lombardy (October). These are the big one day races with a history going back to when Moses was a little boy. Win one of these and you are a legend. In my book all are of equal prestige, though an Italian will lean towards Milan-San Remo, a Belgian to the Tour of Flanders etc.

There are also some other races, like the Paris-Tours and the Amstel Gold race which are considered 'classics'. Some races, like Paris-Brussells, were once classics but for varying reasons have lost prestige. Other races, like Gent-Wevelgem, Fleche Walloone or Het Volk, depending who you talk to can be considered classics or semi-classics.