Depends on the project you are working on....for commercials, where you have seconds to show what you can you can use everything..Raytracing....even GI, Caustics, because rendertimes for a few seconds are not as important. In this case, i'd prefer Mental Ray. For Movies and full animated movies rendertimes are more important, and in most cases the budget is much higher...in this case Renderman is the choice...a fast scanliner, with fast motion blur, DOF, HighQuality Displacement. BMRT are only used for Frankenrendering..which means if u are working with Renderman, but need real refractions/reflections, where a fake doesn't work. Otherwise BMRT are far too slow. Entropy is a good solution, somwhere between Renderman and BMRT, but support is missing after Exluna was bought by NVIDIA.
Any further questions ?
b u l l e t