Not sure if this counts but Final Audio E3000($50 or $60 tiny little IEM with attached cable) were my gateway drug into hifi and after they broke I'm constantly tempted to buy a new pair to compare with the gear I own now just to see if they still hold up.
But the big ones are probably Phillips shp1900, literally $15 plastic headphone that I still use for watching youtube videos in bed despite the earpads starting to disintegrate. They're extremely light with no clamp at all while at the same time being pretty sturdy in their simplicity, so it's perfect to just put on before going to sleep then toss aside with no care. The sound is also quite laid back. Funny thing is I looked up measurements for the first time just now to see how bad they are, and they seemingly have a large 5kHz dip:
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And now this got me wondering if years spent listening to those is why I like my Elear with their similar 5kHz dip so much.