One of the transistor's main function is amplification therefore for amplification the transistor should have a highly doped emitter region and lightly doped base region so that more charge carrier emitted from the emitter region then recombining with base region such that base current is small after that all the left out charge carriers move towards collector region which itself is moderately doped causing the amplified output. Thus, more doped the emitter region better the amplification.
Hence, emitter is highly doped than collector and base is least doped.