Headphones are a particularly personal item of gear, and to ask for opinions on which are best is an ambiguous question, you’ll get near as many opinions as there are makes and models.
If your interested, I use AKG K271MkII (Closed) for tracking and K601 (Open) for mixing.
I’m not however going to try and persuade your decision, by telling you to get them too.
I recommend to anybody that they actually find a store with a good selection of namebrand “studio” or “reference” fullrange phones and try them on for a while and listen to some reference material that you are familiar with (I’d be leary about buying them online!) Ideal would be if you could take a couple of models home with you to try and then return the ones you decide against.
Remember always however, that regardless of what you use (even monitors) the key to mixing is in the translation to other reproduction configurations. Always mix down a test version of your projects and listen to them on other systems (livingroom HIFI, in your Car, MP3 player on earbuds, at the beach or in the garden out in the open etc.). You have to get to know your chosen monitors, and because of their very nature this is even more critical with headphones.