I have an assortment of headphones here, none of which are what I'd consider high-end, but each is useful in its own way for different applications. For mixing, the ATH-M50s are far and away the best of the lot, not because they have great isolation or extreme high-fidelity (they have neither), but because they translate well.
Among my collection are a pair of Sennheiser HD280's that JonBuoy linked to above. Sorry, JB, I have to disagree with that recommendation - for mixing, anyway. As you noted, their isolation is very good, which makes them a great choice for tracking or airplane listening. But they are unfortunately pretty useless for mixing, having a very uneven high frequency response and nothing at all on the low end. I have tried using them for mixing in the past, but when I'd subsequently switch to speakers I'd be dismayed that I wasn't even in the ballpark.