What is the part of the cerebrum responsible for complex thinking such as memory, speech, emotion, planning, and reasoning?

The cerebrum, or cerebral cortex, is divided into two hemispheres (left and right). Each hemisphere consists of four sections, called lobes: Occipital Lobe, Temporal Lobe, Parietal Lobe and the Frontal Lobe. The answer to your question is  the Frontal Lobe which is located under the forehead. The frontal lobe controls reasoning, planning, voluntary movement, and some aspects of speech. The prefrontal cortex is the part of the frontal lobe right behind the forehead. It is associated with complex cognitive skills such as being able to differentiate among conflicting thoughts, determine good and bad, identify future consequences of current activities.