What parts of the brain are most likely affected by alzheimer's disease?

Medial temporal lobe. Alzheimer's starts in the hippocampus and the medial temporal lobe. In end-stage alzheimer’s there is widespread atrophy. Early onset alzheimer’s (<65 years old) has a different presentation. There is usually some mild hippocampal atrophy, but the most striking finding is parietal atrophy, with atrophy of the posterior cingulum and the precuneus. The hippocampus can be normal. Hope this helps.
Base, then the rest. Usually starts in the front, lower part of the brain called the nucleus basalis of meynert, but then to the rest of the brain as the disease progresses.
Temporal lobes. Alzheimers is thought to start in the temporal lobes followed by the parietal lobes. As the disease progresses the frontal lobes can also be affected. The brain stem is not affected.