The audio on Amazon Prime Video is not restricted to two channels on Roku devices. I get Dolby Digital+ 5.1 audio and 4K HDR10 video on my Roku Ultra. In fact, Amazon is the only app I use the Ultra for anymore, and it's due to the better audio. For everything else, I am using my Apple TV 4K.
I voted for Amazon because it's the one we have used the most since we started streaming, but I'm not sure I would classify it or any other service as a "favorite". Also, I use Amazon as more of a subscription-based service via my Prime membership, so I am not sure it qualifies based on you not...