Innioasis Y1.
You can install Rockbox on it to make it "better" (subjective I guess) or keep it stock and still use it.
One of the reasons I installed Rockbox on mine are for the themes. I can share a photo to those interested to see what it looks like. Links to everything below!
Innioasis Y1 (they've increased the price since I got mine, was $50 a few months ago)
Innioasis Y1 Updater (used to install the Rockbox firmware, or update the normal firmware. NOTE: You can use both the original firmware AND the Rockbox firmware for a "best of both worlds" scenario!)
Innioasis Y1 Rockbox Themes (I believe this is the git that I got all my themes from, but if it isn't I can provide the actual one later when I am home!)
Battery has been good for me, personally, with Rockbox and a sick ass theme (the Snowoasis Y1 DARK theme). I've got ~5000 songs on it, and it's only taken up ~45GB of space!