Kiseido Go Server (KGS)

If KGS's UI looks like it hasn't been updated in 20 years, that's because it's already perfect. There's no ads, it's purely functional, it does exactly what it needs to do and nothing more. If you click on "KGS Plus" you have the option to spend money on lectures given by human pros. Otherwise it's completely free, and it's still an active go server that's been around forever.