I've got just about all the headphones that have been mentioned on here. I'm with Craig and agree they all have their strengths and weaknesses. However, one pair stands at the front for me that hasn't been mentioned yet. I can mix, master and do whatever I need with them and they sound nearly identical to my monitors as far as the over-all eq curve. Of course I get a little more sub low out of my monitors due to using a sub, but these cans are my choice above all others.
If you can find them, AKG K-240 DF's are by far the best I have ever used and translate perfectly everywhere to me. They are discontinued, but supposedly the ones that took their place are even better. I don't know if this is true or not, but it would definitely be worth checking into. If by chance you can actually get a set of these used or someone has new ones still in stock, even if by chance they may not be for you, they will always be one of the best sets of headphones you own.
One of the things I like about them is, when you feel bass in them, you're using too much. Just like a real monitor mix....when you feel too much bass, you most times are using too much low in your mix. The object is to feel a little and hear a little to where the balance gives you the proper amount of bass. These headphones deliver that in my opinion and everything is crystal clean, clear and nothing is over-accentuated.
Quite a few have recommended the Sennhesisers. In my experience, both sets I own are overly bassy and I would never trust them for anything. Sony MDR's are bassy as well and lack mids...though they are super popular. They are good, loud headphones but I'd not trust them for anything other than to monitor something.
As far as bang for the buck goes, the ATH-M50's would rule the roost but for the extra $70 or so bucks, I'd get the AKG's. They just really have a nice, flat response in my opinion that doesn't cloud my decision making. You may have to get used to judging low end in them at first, but I assure you, highs and mids will be absolutely perfect in all your mixes because they translate so well. Low end will always be tough to jugde in any set of cans you buy....but these are the ones I use and I've had great results for about 30 years. Good luck!
-Danny