The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
She is the most powerful witch in the world (Respectively to BTVS)
But since we have WandaVision it’s hard. To say they are both capable of basically the same things. Except Wanda needs Psionic energy, Willow doesn’t.
Also they’re both profound it Protection spells, willow is also a much more intelligent witch. And person for that matter.
Studying the arts of gods, She was able to combined people, which would help her win against Wanda.
I would stay Wanda might be more powerful in terms of raw power. But willow is also able to absorb others magical powers similar to Agatha, but they didn’t have to be used.
So honestly with the help of Buffy, and Giles, willow would win.