Right now I am looking at the M50s with a FiiO E07K as an amp (maybe overkill on the amp). I have looked at a lot of reviews, and I may have misinterpreted this, but there is not a HUGE difference in unamped quality in the $100-200 range, mostly coming down to personal preference. For this reason, I have chosen the cheapest of the popular $100-200 headphones, so that I may buy a good amp alongside.
I have looked at the HFI series, the Shure 840s, DT series, Custom One Pros, etc.
I am looking for a pair of headphones that is mostly neutral, but with a bit of emphasis on the bass. It should be detailed, good at isolation, foldable, comfortable (for a small head), somewhat fashionable, and have a reasonable soundstage.
Based on what I have seen, the DT series are regarded as the best headphones at sub-$200, other than used and refurbished (I would love to see a $200 pair of D2000s, for example). However, they need sufficient amping to reach their potential (which puts me well over $200), and they aren't very portable.