Sennheiser HD 280
NEVER, NEVER EVER USE HEADPHONES FOR MIXDOWNS AT ALL !!!

I'll second Axel's opinion on NEVER EVER mixing with headphones. Try it : EQ a track on headphones to make it sound good, then try EQ-ing it from scratch (preferrably after some time, so your ears can rest), on monitors. Then listen to both EQ-ed tracks on every speaker set you own (car/hi-fi/anything...) and on other headphones then the ones you EQ-ed the track on. You'll see that the headphone EQ-ed track is way off, sonically speaking, on 90% of sound systems your hear it on (the 10% accounts for the possible coincidence of a bad sound system boosting/cutting, making the track sound good by accident).

Also, check these sites out :
http://www.jbsound.com/mixing-primer.php
http://www.jbsound.com/no-mixing-with-headphones.php