I think it would be my Creative Zen:Vision M (30GB)
I have the identical one in my jacket pocket right now. Ironically the first 2-3 years I had it I didn't use it much because it took me forever to rip my several hundred CDs to MP3s. You forgot to mention that unlike many other devices it supports WMAs, and I have many of them (because my ripper back then wasn't licensed to do MP3s), so to put them on say my phone now I have to re-rip.
The only shortcomings it has are 30GB is too small when you have multiple thousands of songs (most are at 128kpbs but even if they all were they still wouldn't all fit) and after 8.5 years my battery life is no longer so great.
There was a setting in the options that let you allocate free space for a removable HD. Useful back when external HDs were pricey and thumbdrives small, but not for years now. Too slow compared to current thumb drives and I need the space for my music!
The Jedi Master