Unless you're getting 4K material (Roku 4 or one of the new models coming). Then I would suggest at minimum 25 Mbps. Netlfix tops out around 16 Mbps for 4K, and I think Amazon is about the same. However, I've seen over 40 Mbps streaming 4K from YouTube, according to their on screen display. Since I have 50 Mbps, I don't know if that was the highest they go, or if I was being capped by my connection speed.
in any event, YouTube can exceed 20 Mbps even with 1080 material, so more speed is always better
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.