I've tried to find the list someone posted recently, where I found 'new' ones of interest to join, but the search engine here isn't quite up to snuff.
Did find this one, but it doesn't have several that I know of included: https://steemit.com/steemit/@aschatria/resources-steemit-discord-server-list-50-steemit-related-discord-servers
A couple of good ones to consider would be #teamgirlpowa or steemsugas; and for fun I'd encourage anyone to join #steempunksnet and participate in the game.
Otherwise, I'd say look at the bottom of people's posts in tags of interest because there are usually promotions & links for the discord channels they have joined. I have created one for the steempunknet channel and put it at the bottom of my posts (the latest ones).
Hope this helps a bit. :)