which one is cheapest depending on where you can become member. I don't know what professional field you work within and your education level. So you need to tell me some more information before I can guide you.
But you can have a look at our price-table for all 23 A-kasser in Denmark. In the first coloumn in the table you can see who the A-kasse accepts as members, and in the second coloumn you can see the price per month for membership.