Not the compact cassette.
It was an good medium in it's time, it was the job of the time, but at only 90minutes capacity* on something that is easily 10x the size of my music player which holds hundreds of hours, no randomise function, and the ability to spill its guts into the machine requiring painstaking retrieval...
Having said that, I'm fairly sure that the "skip track" function on modern players is the first thing to wear out - back when you had 20 tracks, they all seemed to be just right - there is a benefit to having limited storage, you only store what you really like.
On that list - the Psion 5 organiser wins, hands down, in my opinion.
*Okay, 120 minute cassettes were available. If you enjoyed the added bonus of wow and flutter applied to your favourite sounds, at no extra cost.