My favourite Professor is Professor Oak, simply because he was the best. A classic, very helpful, and lots of knowledge, in my opinion, is what a Professor needs to have to be good at his job. Oak ticks all of those boxes and more. He's had experiences in the past - as shown in the movie Pokemon: Celebi: Voice of the Forest - which means that his knowledge comes from actually being a past Pokemon trainer; instead of immersing himself in books, which is only half the job really.