I'm ranking based on specialization.
>Birch=Habitats
>Elm=Breeding
>Rowan=Evolution/Kukui=Attacks
>Juniper=Origin
>Oak=Pokemon/human
>Sycamore=Megas
>Sonia=myths
>Magnolia=dynamax
Magnolia is at the bottom for obvious reasons. Sonia is second to last, only because she stumbles across a lot of her 'discoveries' by happenstance, and contrive bs. Birch, Elm and Rowan's specializations has always been my favorite of the professors. If I was going to study Pokémon these would be the main three I'd focus on personally. Attacks are also interesting, too. And I like that Kukui is the most hands on out of all the professors. I'd put him on equal level as Rowan.
