ideally i'd like a pair for mixing that is very neutral and flat, and a pair for listening that is more lush with a bit of boost in the bass and trebles. i'm very intrigued by the Beyerdynamic DT880 and DT990 premium models, but there are a lot of good choices. i've heard consistently good things about the ATH-M50 as well as the AKG 701/702 and the Sennheiser HD600.
i've been using Ultrasone HFI-550 headphones since around 2005/2006. they've served me well and i'll continue to use them, but i'm strongly contemplating an addition. something to complement the HFI-550, not necessarily a total replacement.