Nothing fancy, but varying. For local files I’m currently using Lollypop on my Linux desktop and Metro on my Android. But actually I use my Funkwhale server a lot, especially on mobile.
I personally don’t need more than a nice display of the front cover, so don’t really care about the back cover or any other image for my local files.
if you’re on Windows, I’ve really been digging MusicBee, and Foobar2000 is pretty great too~ they both support back cover art, among other types
I really looking for something for the lounge on a big ish TV… what itunesLP promised… maybe Kodi or Logitech Media Server… of course without the appropriate good artwork, it won’t do anything…
The whole reason I am here at MB sorting out my music is to use it in KODI as they are heading into better artwork handling for music. (I deleted a long reply here as it was OT and probably better suited to your other thread - https://community.metabrainz.org/t/what-do-you-use-all-that-artwork-for-and-how-to-do-you-use-it )
Like you I want all my media players accessing real artwork. And I’d only trust Open Source projects for that.