Well Im going to be completely unoriginal (and probably horrendously unfashionable these days) and choose some D-Rok.
For my just starting emperors children army:-
D-Rok: Welcome to my world, here come the noise marines. Yes I actually liked the old school guitar weapon noise marines as well... "When I was young" Heavy metal was the default music of choice for 90% of the GW crowd
For my eldest sons Dark Angels:-
D-Rok: Get out of my way - It was the intro track and perfectly suited to the old space hulk PC game. I was forever cursing my termies for being in the wrong order in narrow passages
Stealers Kiss, again Space hulk - which was all deathwing vs genestealers. That game and the music also inspired my long standing fascination with lightening claw equipped terminators...
