I’ve had very good luck with the Sony WM-D6C, the Professional Walkman. Unfortunately, they’ve become collectable and therefore can sometimes go for astronomical prices. But if you’re patient, you can pick up one for a reasonable price.
It’s a very solid, good sounding, reliable machine. On a lark, I paired it with a good preamp and set of mikes, and made some recordings of the early morning bird activity in my backyard. Paired with a metal cassette, I’ve found the quality to be really lovely. I grew up on cassettes and am biased towards the voicing of tape – lame pun, sorry. But what’s especially nice is the way the tape handles high dynamic, low frequency sounds. It’s lazy of me, really, as I don’t need to worry about where to set the HPF on my recordings.
Be sure not to overlook the cassette tapes themselves: they can very atrociously expensive. Your best bet is to head out to thrift stores and sift through the tape bin for used mix tapes. Do be sure to take a listen before you record over them, as there’s likely to be some very interesting stuff on there.