1 Paris-Roubaix: International field every year since 1896, except for some war years. Queen of the Classics (so say the rider themselves), unique course since 1966 as hard in one day as a GT in 20 days (so say some riders).
2 Milan-Sanremo: International field in the early years, national field in the interbellum years, and international field in the post war period with the help of CDC. Very stable course. The longest in modern days. Had to cope with asphalted roads in the 50's and only needed 2 climbs to adapt (up until what you know

)
3 Tour of Lombardy: Historically speaking I rate it above Liège, which is what this thread is all about here (prestige) but I rate it below for the modern days. Same evolution as the one above, concerning field quality but very unstable course throughout the years, which is why I rate it below.
4 Liège-Bastogne-Liège: The oldest but that's eye-wash. There were barely 3 editions in the 19th century, only 1 professional. Then Liège resumed in 1908 but was only a pro race in 1919 or something and only Belgians competing, not even the best of them. Only in 1951 could we see big stars because it joined the CDC and it wasn't more prestigious than the Arrow before the 70's.
5 Bordeaux-Paris: The oldest, it disappeared but in the more recent years, the race prestige still interested some top guns.
6 Tour of Flanders: As I said, not a major classic at least before the Koppenberg was added. Often favoured second-tier riders despite top guns doing all they could to spread the field. Leman is a good example of this.
7 Paris-Tours: Yes very flat but was a major classic until WWII. And between 1974 and 1987, decent course but reversed (Tours-Versailles, etc.), so top riders could win it at the expense of stability.
8 Walloon Arrow: Got its prestige strictly with the help of the CDC but was more prestigious than the sister race for a few decades.
9 Ghent-Wevelgem: got its prestige in the 50's when Bartali and Kübler raced it. Then joined the Superprestige.
10 Paris-Brest-Paris: the decennial marathon race, still existing (as a cyclotourist race).
Yes Championship of Zurich was also a great one but I hated the last course, in circuits.
Edit: Damn, I missed Paris-Brussels, how could I? But well, I don't know where to insert it. It deserves a mention though.