I've only ever used crtmpserver and nginx's RTMP module, neither of which can be set up with a GUI. I've never tried Red5 or Wowza, but Wowza is not free. Between crtmpserver and nginx with RTMP module, the latter seems to be more user-friendly to me. But that doesn't mean it's easy...it involves stuff Linux server management and compiling stuff from source, so if you're familiar with that stuff, it's not bad at all.