I have extremely eclectic musical tastes so this at first seemed like a difficult choice however there was one that I never get sick of hearing so I'd probably have to say...
Children by Robert Miles.
Regardless of 'mix', I just love this song and can always tune into it.
Aside from that, I'm much like Drawmack in that it really depends on mood. One minute I can be listening to Tori Amos In the Pink, the next minute I'm LMAO off to Limp Bizkit (I really love their music as I think it's mainly comedy and taking the piss and it just sounds good), and then I'll chill to Floyd (Animals is my favourite album) or if I'm feeling pretty dark then Nick Cave and the Badseeds is always a good choice.
I love older music as well and particularly like the music compilations on Pulp Fiction and Good Morning Vietnam. Sinead O'Connor gets me all riled up with Universal Mother but then I'll cool right down with Lou Reed.
Boppy music with a bit of a funky twist lightens my mood so I like listening to Bjork every now and then but if I really feel jazzed up, I'll put on Morphine (such a shame he died, they were damn good).
Then for sheer listening pleasure, I'd say Queen or Massive Attack or Moby or Enigma or Robert Miles (again).
My favourite, all time BAND, however, is Laika. Go check them out at
http://www.laika.org/ as they deserve all the praise and support any new fans can give them. They embrace and reflect pretty much everything I love about music. Oh, and they've got six FULL mp3 samples of their songs. Black Cat Bone is probably my favourite of the free ones.