Sennheiser is awesome, but most of them are open back. That means better sound quality but no blocking of outside sounds (besides whatever you get from having music blasting in your ears). I have the HD 558s. Love them. Had the 555s before and thought the sound quality was even better, but their headband tends to crack. The 558s are extremely comfortable.
For noise-cancellation, I have the Bose Quiet Comfort 15. I found them on ebay for only $110, which was way cheaper than the $220 or so you'll see them listed for elsewhere. The sound is good, though not quite as amazing as the Sennheiser. The main benefit is that putting them on basically blocks out everything. They do have an on/off switch and require an AAA battery, which is kind of annoying. It's easy to forget to turn them off and drain the battery.