Simply, which headphones were the first you ever used? Excluding things like iBuds and the on-ear foam cheapies that come with walkmans, of course.
Mine were the KOSS K/6X Plus. Got them when I was 8, found them at a local garage sale for $5 and made my grandma buy them for me. Didn't actually get to use them until about 2 years later, when I found a (mono) 1/4" adapter. They weren't very comfy but my small ears were very happy with them. They're incredibly sturdy and light; I tossed and tangled them and never managed to break them. I still have them, and interestingly enough I still enjoy them from time to time, they're nothing special but they're better sounding than my M50 and they're pretty stylish.