The? + superlative adjective

1. Mangoes are most expensive in early summer, when they first come out.
2. Mangoes are the most expensive in early summer, when they first come out.
3. Yellowstone is most crowded in summer.
4. Yellowstone is the most crowded in summer.
5. He's nicest when he's with children.
6. He's the nicest when he's with children.
7. She works hardest when she's doing something for her family.
8. She works the hardest when she's doing something for her family.
Which of the above sentences is not acceptable to native speakers?