I've owned a handful of BMWs (no S54 though) but I'm partial so the sound of the S62 V8. The turbo 6 generations were truly a remarkable change for BMW in terms of power, though.

At the moment, I have a Boxster, and the flat 6 somehow feels smoother than any I6 I've owned. I've done some research and sure enough, the inline 6 is (mathematically) smoother than an inline 6. The only way I can explain the fact that the boxer feels smoother is because it's mounted low and behind me, away from steering components? Whereas in my I6 cars, with the engine in front, you feel it through the front subframe and steering a bit.