As said, Yuins for earbuds, but I'm more into portable supra-aurals. Here goes:
It depends on what you mean by "portable." I don't consider headphones like Denon D1001 or Alessandro MS-1 (or Grado SR-60/80) portable. They're just too big, too cumbersome. The ESW9 would be on the large side of portable for me. That said, here are what I think your live options are:
1. If you really want noise attenuation, AKG K414P can be had for ~$60. They don't get a good rep here because many think the bass is boomy and overpowering. However, when used in noisy environments the bass overemphasis is a godsend. Honestly, there aren't many isolating headphones in this (price and size) category. The JBL 510 is supposed to be really similar, and for cheaper. Caveat emptor.
2. ATH-SQ5 sound much better than the K414P, but come at greater cost. A little over $100. They're clearer and have a more neutral presentation than the K414. But that's not hard to achieve, really. However, they're less portable. I wouldn't throw them into a backpack. They feel cheap, plasticky. They're light and comfy, though...if they fit well. The odd shape doesn't please all ears. These are also closed and provide decent noise attenuation if you achieve a good fit.
3. PX100s are the best open-backed portable option for your price range, IMHO. They sound great - I prefer them over the SR-60 (I think...), but not over SR-80 - are very light and pleasingly small. However, they feel cheap and plasticky too. I'd worry about tossing them in a bag. They don't attenuate outside noise at all, which is a pain.
4. Way, way down the ladder, KSC75s are the way to go if you like ear-clip headphones. They don't sound much worse than the PX100s, really, and they're super cheap. Like $5 sometimes. Crazy.
5. iGrados would be the behind-neck headphone of choice for me, but they're a little heavy and clumsy if worn with glasses. They're definitely better than Sennheiser's lower end behind-neck options (PMX-60? I forget). They sound pretty much like SR-60...I don't think I'd be able to differentiate the two. Those with better ears might.
