I swear you people are all the same, this thread has turned into a praise m50, or promote your own favorite headphone thread.
For the money there are many options: sr60-125i, m50, shure 440-840, m50, ad700, k240, k272, d1001, used dt770 pro, srh750dj, senn hd558 or hd555, etc.
Yet you people continue to make blatant statements saying 1 is the best choice. This is the kind of hype this thread was trying to illuminate, only to fall victim to the same hype along the way.
In terms of sound signatures, there is a better way to classify these.
Mids/Highs centric - sr60-125i, hd558 (or so i've heard), ad700
Smiley Curve (Bass and high centric, slightly recessed mids) - m50, dt770, d1001, srh750dj
Generally Neutral - k240, k272, srh440-840, perhaps hd555 and hd558
Also for those preferring the smiley curve sound signature, as I know a lot of you do, I preferred the d1001 over m50 in that genre.
Out of such headphones I personally have heard: sr60i, m50, d1001, k240 and hd555. From those choices, and according to my own preference, I prefer a modded sr60i. But there are lots of options and I am by no means going to tell you they are the best in the $100 bracket. Choose based on your preference, and then what you think is right for you.
Hopefully this post will bring some sanity into this downward spiraling thread.
Edit: Please note this post is not meant to offend, only inform.