Very hard to pick one from all time what with all the different genres etc.
Rock and Roll, Purple Haze - Jimi Hendrix
Dance wise, Hardfloor - Acperience
Chilled out, Unfinished Sympathy - Massive Attack.
Metal, Walk all over you - AC/DC
Classical, Holst the planets or maybe that old spice ad (cant remember the name)
Bubble gum pop - Saturday night - Whigfield
of course, if you ask me this tomorrow it'll be totally different.