I'm with MVF4 - one more vote for the S54.
I've owned the E34 M5, E39 M5, and E60 M5 - of those my favorite motor is the E39 V8. High torque allows excellent drivability. High torque also means monster burnouts! The V10 is pretty hot, but has relatively low torque, narrow powerband, and consumes gas like none other.
However I think the S54 is simply perfect; great horsepower and torque for its displacement, very linear powerband (allows driver to finely control the rearend), beautiful sound, and reasonable consumption. It's pretty much the best NA I6 motor ever built.
Here's a photo of the S54 from my old MC.
