So far I have used:
Ouya - XBMC
Raspberry PI - XBMC (2x A+, 1x B+)
3x Roku - Plex, PlayON, Amazon Prime, paid channels
2x Chromecast - just YouTube
Fire Stick - Plex, Amazon Prime
Win7 Atom HTPC - WMC, XBMC
I have a cheap quad-core Android media box coming to play with.
So far, the best and most useful has been the Roku. I do like the Fire Stick also, but it doesn't have as many channels. The RPI actually had pretty good XBMC, but just can't handle Dolby stereo and 1080p video very well.
So each one of these have their merits, but on a daily basis I use my Roku and Fire Stick. I don't use anything but Amazon Prime so I don't know how Netflix is these days. They lost me as a customer forever.