Just thought i'd comment in on this thread because i've messed about with a lot of this my self and went through a lot of the same problems in choosing headphones.
First thing I want to get out of the way is that headphones are NO GOOD for mixing. Period. Can you mix with headphones? Sure you can, and with enough practice you can get good sounding mixes too, however, my point is that headphones are poor choices for mixing, no matter the quality of them. You will likely find that when you mix things on headphones, things might sound perfect, but once you bring the same song to speakers in an open room, the levels will sound completely out of whack. Professional audio engineers will almost never mix using headphones and its for good reason! With practice and experience you can learn to counter the short comings of headphone mixing so that they sound good on speakers, but it's annoying and time consuming, but if you are on a budget it is an option.
The primary purpose really, for using a pair of headphones is for monitoring (as a musician) or for critical listening in the mixing or mastering process in which you need to hear very subtle sonic details or differences that can get lost on speakers in a room (headphones offer better up close "resolution" I guess you could say). In reality, to produce high quality mixes you need both, good headphones and good monitoring speakers, that isn't to say that you can't get away with only one or the other, but having both opens up more options and makes things a lot easier.
Now, that said, there is no magic headphone that will be better than everything else out there simply because sound is open to interpretation and is highly subjective to ones own opinion (as all things in the music industry are, and I think the number of responses in this thread suggesting different 'phones is proof of that).
Just remember that just because something is expensive does not mean it is good, this is a fundamental problem with the sound industry because people pay so much for something that they start to hear only what they want to hear in order to justify their purchase. I think i've said this elsewhere on the forums, but i've seen blind tests conducted on other websites where $3000 professional studio mics were put up side by side with home-studio level $150 - $300 mics (running the same pre-amps mind you) only to have most users say the cheaper mic sounded better to them, so take from that what you will as it applies equally to headphones.
When looking for headphones you've got 2 things to look at. First off whether you want open back or closed back headphones. If you're going to do a lot of recording using a mic, you're going to want closed back headphones as they'll prevent sound from bleeding into the mics as you record (these are typically referred to as "monitoring" headphones by most manufacturers). The draw back of a closed back set of cans is that they can be pretty dead sounding and will lack the sparkle that open backed headphones provide. The benefit of a closed back set of 'phones is that they do tend to have a higher bass response and excellent isolation which can be helpful in pin-pointing problem areas sometimes in a mix.
Open back 'phones on the other hand allow the sound to breathe, these types of headphones are no good for any sort of mic recording (or even listening with around other people 'cause they'll hear everything you do ;)), however, they provide MUCH more natural sound as sound is free to travel to both ears from either earphone making the sonic quality you perceive higher, more detailed and more airy. If you insist on mixing/mastering using a set of headphones, you're going to want to look for an open ear pair as this will give you the best results in my opinion.
The other thing you'll want to look at is frequency response. While this won't really tell you anything about how the headphones actually sound (again, like I said, it's subjective), it will tell you the range in which the headphones will represent available frequencies. The most common for most studio headphones from the majority of companies is probably 12/15 to 25,000 Hz, this is usually adequate for most people in most cases. If you move to the top of the line of most professional headphones you will typically see the range go from 10 to almost 40, 000 Hz, probably overkill unless you love to spend money or you're doing this professionally.
Personally, i've used both AKG and Audio Technica headphones, both sound good, both are different style headphones and both serve different purposes for me, and between the both of them I probably spent less than $250. Stick with a reliable company (there's been a lot of good mentions here already). Try to avoid stuff like skullcandy or bose (this has already been said), not because they're bad headphones, but because they're consumer grade and not suited for recording use, they have very skewed frequency responses which makes them great for listening to music but difficult to use for mixing purposes.
Grado, Sennheiser, Sony, Beyer Dynamic, AKG, Audio Technica all make excellent 'phones with fairly neutral frequency responses suitable for recording/studio use, and they all offer models that won't break the bank.
I know this was long winded but I hope it was some help as I was once in the same position you were in.
Regards,
Daniel
