I've always use https://www.gokgs.com/
quite a lot of people at the time I was playing, with lots of different room (by country, interests, level, etc..)
But I'll definitely give online-go a try ! (interested to see a well made software about go ! KGS one is quite old school..)
Sadly these days there are not so many players on western servers in the higher ranks. But I like the software the most. It's made by one of the github founders I believe.