Depending on your budget and type of music, there are many quality headphones that you can purchase for your pleasure. Anyway here are my recommendation for following price range:

Under $50 - Koss Porta Pro
Under $100 - Grado SR 60
Under $250 - Grado SR 225 (best for rock&roll type of music)
(Grado generally feels little uncomfortable for me since I have BIG ears but with slight modification of headbend, they can be quite comfortable)
Under $300 - Sennheiser HD 580 (this one you can buy around $200 street price now since it is no longer their top model)

Best sealed ear-canal headphone - Etymotic ER4S (this one gives you 23db isolation, great for commute, air travels, and noisy environment but requires quality headphone amp to reach full potential)

And if price is no object and you love classical and jazz music, you might want to look into electrostatic headphones instead of dynamic headphones. Generally they are very expensive.

If I wanted to just plug my headphone into any available jacks (990, portable, computer, iPOD etc.) without the headphone amp, Grado SR60 will be my choice. However, for my 990 headphone listening, Sennheiser HD580 gets most usage and it sounds great. Try to audition as many headphones you can get with your favorate musics. Hope this info helps you to make informed buying decision.