Well if you can find Alpine White with black interior I think that is the best looking x5. You can really get that "storm trooper " look going that is sporty but classy.
I also think the aero kit looks best on Alpine Whie cars. I could never find a white one optioned the way I wanted it so I went with Space Grey, and don't regret it, but I am partial to the look of a Well modded white car.