Grade A (Superpower): USA
Grade A- (2nd Superpower): USSR
Grade B+ (Major power+): Russia China
Grade B (Major Powers): Germany, France, UK, Japan, India
Grade B- (Middle Powers+): Canada, Australia, Turkey, S. Korea, Brazil, Italy, Mexico, Spain
Grade C+ (Middle Powers): Iran, Indonesia, Netherlands, Saudi Arabia, Poland, Norway, Sweden, Denmark, South Africa, Nigeria, Malaysia
Grade C (Middle Powers -) Pakistan, Vietnam, Phillipines, Thailand, Ukraine,
My own personal list that is not based on any academic studies, so feel free to critique. China and Russia are simply not superpowers, but they are on a different category of major powers due to large military, unsc, economy, resource, political influence over neighboring states, and most importantly an independent foreign policy, (the other major powers are in USA camp).
We can agree that Italy is not a major power, but it is far more influential than Vietnam, hence my use of + and -.