Tour
Ronde / Roubaix / World Championships
La Doyenne / San Remo / Strade
Giro / Lombardia
Vuelta / San Sebastian / Gent-Wevelgem
E3 / Omloop / Flèche Wallonne / Amstel / ...
Something like that would be my order. But it depends on the person in question of course. Like today is of less (if at all) importance to Roglic who'd have the Tour by far and away at #1, but this week is way more important for someone like Alaphilippe who knows he can't win the Tour (although being French it's different for him as well).
Maybe the three Grand Tours would be 1, 2 and 3 if you ask a starting rider which races he'd like to win if he had unlimited potential, but for me the Ronde and Roubaix carry a very big importance as well, and competing in the rainbow jersey for a year is a unique and great experience.
It's basically the WC, Grand Tours and the very big one-day races though. 1 week races like the Dauphine, Switzerland, Paris-Nice, Tirreno, BinckBank, ... are "nice to haves" but they're never the main goal of riders before the season starts. Let alone the lesser known ones like Belgium, Turkey, Poland, Germany when they still had one, Algarve, Catalunya, ...