Well, it mostly depends on what you mean by "Top Professor". In my opinion, there's no thing as "Top". There's the most influents on the scientific community (e.g. in my field, if Milton Packer says something about Heart failure, you're more prone to trust him because of his resumé), there's the good teachers, the ones with outstanding human qualities et cetera.
Here's some hints:
If you're looking for the ones that have the most impact on the scientific community, you could go and search the topic you're interested in (e.g. "Placenta Previa") on google scholar and then look at the authors of the most cited articles (this method is far from being perfect because citations can be biased in many ways).
One other thing you could do is searching for guidelines on the topic you're interested in and looking at who wrote them. For example, these are ESC Guidelines on Cardiovascular disease during pregnancy.
Then, if you're curious about who gives the "best" lectures (better: the ones you like the most), you can search on google for congresses where they were invited and give a look at their lectures.
I tried to give you some directions but bear in mind that every method is not perfect and there most certainly are very good professors, super skilled at teaching, that are just average researchers and not incredibly influent.