AKG K55

Those were my first headphones. At €35 they were affordable and my digital piano sounded way better than through my bad keyboard amp, but that was about it. I never listened to music to those, I liked my small hifi system better.
 
Coming back to them after a few years of storage, I was surprised that they sounded decent when simply listening to music. They had a rather pronounced bass and treble range, but otherwise they were rather balanced. Also, they are light as a feather and therefore pretty comfortable - almost too comfortable, though, because the clamping force on my head was way too low, which caused them to slide around.
I also found out that with higher volumes or high dynamics (like from a digital piano) they distort rather easily.
 
So, when I had a little collection of better phones, it was pretty easy to let this one go.