Audio-Technica ATH-M50s have long been a standard, and they are my workhorse cans for detail editing. That said, if I had to do all my mixing on headphones they would not be my first choice because they are far from flat.
To be fair, headphones generally aren't flat. It's physics; you just can't enclose tiny speakers in a tiny enclosure and expect them to be. But an open-back design avoids some of the acoustical limitations and will yield a more realistic frequency response, so that's what I'd recommend for mixing.
The downside of open-back headphones is they don't isolate you from outside noise, nor from your own sound leaking out. They are therefore not practical if you have to work in a noisy environment, or if you want to mix in bed while your partner sleeps. But assuming those concerns don't apply, look at the AKG K240 MkII or the Sennheiser HD-650 (expensive).
These are what I use for critical listening:
http://en-us.sennheiser.com/audio-headphones-high-end-hd-558Not exactly high-end, but relatively inexpensive and have a better-than-average frequency response and detail. Plus they're extremely comfortable and rugged.