The iRiver MP3 players were fantastic throughout most of the 2000s (unfortunate choice of name, they existed pre-iPod). They were the player of choice for the technical crowd because they supported line-in recording, optical outputs, and OGG playback (which mattered a lot more when capacities were smaller). They had an average-quality interface, usable but not amazing.
They were also one of the main bastions for the open-source Rockbox firmware, which supported a huge variety of codecs and WAV recording. Rockbox's appearance was utilitarian but it actually featured many great usability tweaks (exponential scrolling/FF/rewind, "car mode" that paused playback when power was lost, bookmarks, etc). They were an amazing player for someone who could do hardware mods. I kept mine going for a decade by swapping parts. I eventually used an adapter to plug in a CompactFlash card for a primitive SSD, and installed a high-capacity battery, which boosted the battery life to something absurd like 20 hours IIRC.
On that note I still don't actually know anything that can touch the battery life of the early iRiver flash players (iFP series). We're talking 24 hours on a single AA. I've actually been looking for a replacement with those same characteristics (extreme battery life on a single disposable AA or AAA battery cell) and coming up empty. You would really think there'd be a successor using modern electronics but everyone has gone to integrated batteries. Anyone know of one?
The Creative Zen Vision M was another really solid attempt. The scroll-strip was a worthy competitor for the (patented) Apple scrollwheel. Nice fit and finish, just couldn't make any waves in the market.