We reckon the Shure SE535s are the best sound-isolating headphones on the market. They're extremely pricey but they're absolutely worth the money for audiophiles. If you're strapped for cash, you might consider chasing down a pair of the older SE530s, which are now available for around £250.
Incredible sound quality; Comfortable; Replaceable cable; Plenty of accessories included
Extremely pricey; Over-the-ear form factor won't be to everyone's tastes