1. May I take your order?

2. Would you like to order now?
3. Will you order?
4. Order, please.
5. Please order your food.
6. Are you ready to order?
7. Do you want to order?
8. Why don't you order?
9. How about ordering?
10. What do you think of ordering?
(In a restaurant, can the clerk use those expressions to get the order from customers?)

The best sentences are 1, 2, and 6. The others sound a little rude.

Not true.

Yes, in a restaurant, the clerk could potentially use any of those expressions to ask the customer for their order. These are all polite ways to prompt the customer to let the staff know what they would like to eat or drink. The specific phrasing may vary depending on the restaurant's culture or the individual server's style, but the general intent is to convey that the staff is ready to take the customer's order.