My experience: Hong Kong, Japan, Singapore, NA, KR (every region high elo diamond-high immo except NA because I get 150-160 ping - I have lots of radiant/Immortal friends in NA though and I watch their discord streams somewhat regularly and am gold-plat-ish elo on the server w lots of Match MVPs)
Hong Kong: Lots of filipinos, and super toxic. It is crazy how toxic the people are on this server, I really don't recommend. Very disjointed strategically as the comms are lacking either substance or at all. The server itself gets weird packet loss sometimes. I know a lot of Korean high elo players play on this server because it's freelo.
Japan: freelo. people are bad mechanically and strategically. you can just aim diff tbh. they are really nice people though. KR players also farm this server.
Singapore: Pretty good quality for the most part. I get 80 ping but because i can rely on my teammates more than other regions I can farm diamond lobbies just as if I were on lower ping, the comms are decent - on average better than HK. The util usage is good too.
NA: The bad players are really bad, but the good players are crazy good. Like insanely good. There's a large playerbase so for the most part they are what you'd expect when you see their rank.
KR: Can be quite toxic in low-mid elo. However from experience they have the best understanding of util usage in ranked and have some crazy aim demons. https://youtu.be/InBCtuz1Yw8
I have yet to really meet someone who has both though lol. Also their gamesense is not as good as NA.
I also played Mumbai once. Nothing really special. Not particularly toxic not particularly crazy.