Sony MDR V6 Studio Monitors- clean, real, un-colored, full spectrum sound. Similar to the Sony MDR 7506 suggested above, but I like the reproduction in the lower end better. These have been a reference for a long time. About the same price, but beware of knock-offs. It's better to get a used set in good condition if you want to save a few $$, can be had for around $65, and replace the earpads, as they are so soft and conforming that they only last a few years. The good news is new pads are only about $8/set.
I also agree with @seamonkey above, that's it's a good idea to also have a couple/few sets of lesser phones, as well, because not everyone listening to the music will be wearing good Studio Monitors, so you need to know what they'll be hearing.