To me, the best music to listen to while playing poker online is Heavy Metal/Alternative.
System of a Down, Ministry, Marilyn Manson, Nine Inch Nails, Gravity Kills, Sisters of Mercy, etc.
Old AC/DC with Bon Scott is my favorite to listen to while playing cards in general.
Whatever music you listen to...as long as your listening to music, it's all good!
Metal, Alternative, Rock, Hip-Hop, Rap, Reggae, Classical, Jazz....turn it up!
There are times when silence is best, too. I guess it all depends, doesn't it?
