I am strongly biased toward the Roku line/ I use my Ultra for 97% of all my viewing. The reason is not so much that I really like the Ulrta's interface but rather more that I despise the Fire TV's interface.
I have found a good use for my Fire TV. The one I have has a optical output that I can convert to regular audio and therefore I can use the Fire TV without actually having a live connection to a TV. The Fire TV also has the ability to be controlled directly by my Echo devices. So I hook the Fire's audio to my whole house audio and use it to play my audio books. I can hear my audio books wherever I am and, since I have multiple Echos I just need to say Alexia (or Echo) play or Alexia pause and the book I am listening to responds correctly.
But that is the only good use I have found. Otherwise the Fire is nearly useless for me.
The Roku Ultra, and in a couple of other rooms Roku 3s, are the only devices I use for watching TV. They are almost too easy to use.
I should qualify this a bit: I do NOT use any "advanced" video or audio functions at all. I watch everything at 1080p or less if that is what the video is in and I use nothing but stereo audio. My old eyes and ears are not capable of hearing or seeing "better" options. Therefore I cannot testify as to the Roku's abilities in those, so called, advanced areas.
I also have a Shield TV pro and a Raspberry PI and I like the Rokus MUCH better than anything else I have.