I don't think you can say that someone is definitively "the best" at doing something like studying Pokemon. As an example, Issac Newton discovered gravity, Albert Einstein discovered relativity. Which one wins at physics? There's not really any way to tell for sure.
It does make sense that Professor Oak would be the best known, however. His Pokedex is the kind of thing that would attract a lot of attention and receive a lot of publicity, while Professor Rowan could make a dozen breakthroughs in understanding how and why Pokemon evolve and still not be as well known. These kinds of breakthroughs might revolutionize Pokemology, but no one who isn't, themselves, a professor will be able to understand that.
On the other hand, if Professor Rowan discovered how to synthesize Moon Stones or something similar, he'd probably be skyrocketed into fame, because now he's got something to show off to the press.
Incidentally, can I ask why being serious makes you a better Professor? Why does it matter whether you do your job with a smile or a straight face?