Well, I'd mention Hypixel, Mineplex and The Hive, but they're all pretty overrated. I'm not a server expert, so there are tons of good servers out there that I don't know about... However, there is a good Steven Universe server known as "Gemcraft", although some people can get away with swearing if you're not comfortable with that. There's also a Pixelmon server but I forgot the name of it.