1. Lossless playback. I am willing to / currently pay for it. I don't care if it's a placebo--I think I can hear the difference when I'm listening through good headphones and a reasonable quality DAC. Plus, some musicians like Neil Young have made this a not-odious condition for being on a service.
2. Best effort on catalog. A huge catalog isn't actually super important for reasons stated below. Amazon's catalog is a little on the sparse side, but the others' are fine.
3. Allow personal user-uploaded tracks, and stream them back to the uploader account losslessly. This allows me to backfill the crazy and not so crazy stuff I've purchased over the years that is not on any of the streaming services.
4. Support a reasonable number of devices such that I don't have to choose between Prime Video and your streaming music service on my TV (to which my hifi equipment connects digitally). Right now I'm running Tidal (best compromise for me) sideloaded on a FireTV.
Or distilled to first principles: "Allow me to enjoy the convenience of streaming music, while allowing me to access any content I wish (even if I must pay for it), at the same quality I experienced when I was 16 shopping at the local record store."
Nobody does this.
Google Play Music: Not lossless; doesn't support FireTV because arbitrary corporate bickering reasons. Props for self-uploaded tracks.
Amazon Prime Music: Not lossless (worst encoding). Props for self-uploaded tracks.
Apple Music: Not lossless; limited device support. Props for self-uploaded tracks, however the interface to self-uploaded tracks and streaming music not really well integrated on AppleTV.
Spotify: Not lossless; no self-uploaded tracks.
Tidal: No self-uploaded tracks.
Deezer Elite: Only available in lossless on Sonos. Props for amazing catalog and self-uploaded tracks.
In terms of fixing things...
Amazon Prime Music could be my choice overnight by going CD quality (not like they don't have the infrastructure to do it). Their device support is surprisingly good. Their catalog isn't great, but I'm willing to buy / upload CDs that are missing--provided Amazon doesn't start shitting-up their content availability to drive CD sales.
Google Play Music also not too far off mark. Fix the lossless thing, and I might be convinced to live with their Amazon bickering.
I suspect Apple will launch lossless soon, but also think it will be a cold day in hell before I can use Apple Music on something like FireTV. Also, the integration of self-tracks and streamed-tracks is bad and should feel bad.
Deezer Elite: OMG, don't make me buy a Sonos. I want you.
Tidal: I like jazz, and your partial Pat Metheny albums and anemic Dave Holland selection kill me. But if you allowed me to upload my own, I could buy my way around your content negotiation failings.