This question will never have a definitive answer but I thought I would like
to hear what the peanut gallery has to say about it.
Lombardia is my favorite but the calendar move means it can't really compare
in importance at this time. Milano - San Remo and Paris - Roubaix get more
publicity and each have their own challenges. I think that the Tour of
Flanders has enough cobblestones, hills and tough on form competitors to
make it probably the most important and difficult to win followed closely by
Liege - Bastogne - Liege. AFAIK they each have the same points awarded in
the UCI ranking and World Cup.
Tour of Flanders
Liege - Bastogne - Liege
Paris Roubaix
Milano - San Remo
Giro di Lomabardia
I suppose a follow up question would be; what classic deserves to be treated
as a monument that isn't? Which (if any) of these five don't deserve to be
on the list? I like Fleche Wallon and I always have to remember that there
are no World Cup points for it. Clasica San Sebastian has an excellent
course but not a long enough history to be considered a monument.