ATH M-50.
 
You have to get to know them and then they are very hard to replace.

Vastman
I've been using KRK's KNS-8400 for years and love them... Sonarworks headphone calibration software has profiled the 8400's.  There's  pretty much universal agreement this improves the quality of hp mastering...  

I would say exactly the same thing as Vastman.  Same goes for me.  All of it.  What he said.
 
But - I'll add this.  Whether you go with the KRKs or any other headphone - look for one (like the KRKs) that is an acknowledged, widely used set.  The reason is that supporting software like Sonarworks, or say the reference tool of a Focusrite VRM box (which is also very useful) have these standard units as part of their presets and modeling selections  That's really convenient and helpful.  Not only the KRKs, but a few of the leading ones (I think one or two mentioned above) have that going on in those types of supporting packages for reference monitoring.

You can't go wrong with a pair of AKG's K240 studio's, they sound great, are very comfortable, and easy on the wallet at $99.

The AKG K240 is an excellent choice in an over-ear headphone that you can wear a long time. Great if you wear glasses and replacement parts are easy to get.
 
And yes, they sound great.

Another +1 for the AKG K240's.  I've got a pair of the older 200 ohm cans and recently picked up the newer ones which are 55 ohm and detachable cable.  Paired with the Sonarworks Headphone Calibration software makes them very nice and you can get both for less than $200.  I can't speak for the M50's but I do have the M40's and like them for extra tracking cans.
 
AKG K240 - $69 @ BHPhotoVideo or Amazon
Sonarworks - $99 (3 pack downloadable) for K240
or get the ATH-M50x and single calibration for $206

TheMaartian

Sir William
I use the Audio Technica ATH M50 and they're superb. Over the ear, very comfortable in long runs. I wear glasses too. Sonically excellent.

Quotes like that are the reason those phones are on my wish list! 

I own two pair of these and they are great for tracking and roughing a mix. You'll still want monitors to check the mix and finalize stereo placement of certain elements, but you can go a long way towards a finished mix once you get familiar with how things are supposed to sound in your headphones.
 
Regards,
Dan

Sennheiser HD 280 Pro
Can't beat 'em.

Just caught the Groupon for ATH M50X for $99.  I've been hearing nothing but praise for these headphones, and the removable cable feature in this model makes it even more appealing.

I USE THESE:
 

http://www.akg.com/pro/p/k240mkii
 
 
THEY are ON ear....
but work for me just fine, wearing glasses sometimes...

I use the Audio Technica ATH M50's for tracking  And the Beyerdynamic DT880's for mixing.
 
If you keep your eyes open you will eventual spot these on a killer sale somewhere.
 
Good luck!