Definitely the S54. Besides the fact its from one of the best M cars in my opinion. It has awesome Horsepower from an amazing sounding 6 cyl. It looks awesome in the engine bay and the car performs unbelievably well. Plus it doesn’t take away from the car itself. You can have the best show car and the S54 will look right at home. You can have a Drift car the S54 will do wonders.
Unlike the M50 it also adds value, its not just swaping an engine from another bimmer.