Here's how I would organize them into tiers:
Genius: Robin, Nami, Franky
Smart: Sanji, Chopper (excellent at medicine but too naive), Usopp, Brook
Average: Zoro
Dumb : Luffy
Here's my ranking for them (not completely confident with some of these):
1. Robin
2. Nami
3. Franky
4. Chopper/Sanji
5. Sanji/Chopper
6. Brook
7. Usopp
8. Zoro
9. Luffy
Genius: Robin, Nami, Franky
Smart: Sanji, Chopper (excellent at medicine but too naive), Usopp, Brook
Average: Zoro
Dumb : Luffy
Here's my ranking for them (not completely confident with some of these):
1. Robin
2. Nami
3. Franky
4. Chopper/Sanji
5. Sanji/Chopper
6. Brook
7. Usopp
8. Zoro
9. Luffy