It would be a toss up between the Sennheiser 650 and Beyerdynamic DT800 600ohm. I like the fact that each have a gradual taper starting starting at about 1kHz as opposed to Grado which have peaks in the midrange and upper midrange/treble. The 650 and 880 will give you a tonally accurate, natural sounding headphone opposed to a more bright and forward sounding Grado signature.
Each of these headphones will need a headphone amp of some sort with exception of the Grado's.