In some cases, the torp of a line uses a better blend - but not always. It really depends on the particular line for me. But I do know that in some cases the blend in the torp is better on purpose such as the La Aurora Cien Anos(actually called a belicoso but it's the same idea). But with some lines the corona is better than the other vitolas. The La Aurora Maduro #4 is clearly(IMO) better than any of the other vitolas of that line, including the torp. Sometimes the churchill is best because a blend needs the extra time for the flavor to develop. CAO Black comes to mind. So for me it's all about which gives me the most pleasure and the actual shape has little value outside of that.
torp ??? corona
robusto ??? belicoso
churchill ??? petite corona