Favorite obsolete formats.....

It wasn't a medium, but a source.....the lowly hand-held transistor radio that only played AM, but introduced many of us to the world of modern (for that time) music. Elvis, Chuck Berry, Little Richard, Fats Domino, Little Eva, along with many others that kept us up way past bedtime until the battery died. The fancy ones had leatherette cases, the sound was as low-fi as you could get, but it was a passport to rock 'n roll nirvana and cool personalities called DJs.

Edison wax cylinder, of course!

I add a vote for multichannel SACD.

Everything analogue is what I miss. I have a finely tuned and customized Nak 682ZX that I play every other day. I make compilations off vinyl and cds. And I use $2.4k retail interconnects with it. And the very best cassettes - Maxell Metal Vertex. Most people have no idea how cassette deck can sound. No, it's not good R2R or comparable turntable. Biggest problem - you got diminished soundstage, but it's not bad.

KDKA Pittsburgh...AM talk radio. First AM station.  Powered up to 50,000 watts each night. Listening as a kid in bed it was another world. Clear as bell on a good radio, but you could listen with a 5 cent transistor and a few wires.