The specialists at CycloWorld have created an index that is ideally suited for comparing the severity of bikerides.
CWiX 500 : the list of the most difficult gran fondos
CWiX 250 : the list of the most difficult sportives
Gravel 500 : the list of the toughest gravel gran fondos
Gravel 250 : the list of the toughest gravel sportives
XCM 500: The toughest MTB marathons
XC 250 : The toughest MTB sportives