Openstore is maintained by Sesame (Dave Lee and Cathy).
I have used Live Storefront (Mandeeps) which works great for me.

I would have a look at nopCommerce - not a DNN module, but very powerful and free,

Happy DNNing!
Michael

When it comes to e-commerce, I'm afraid that what is "best" is very much up to interpretation and also a moving target with matching things up to any requirements you might have.  

The options provided all have their pros and cons, as will any other suggested solution.  Since we don't know any true requirements, we can't truly say something is the best choice for you to start with.  

In addition to what was suggested by the others, our company maintains Hotcakes Commerce.  It's generally one of the first extensions in the DNN ecosystem that people look at.  

@Will 

Does Hotcakes have a Mollie payment?

Hotcakes Commerce has a payment provider feature that allows organizations to build and include any payment method or payment gateway people wish to use.  Mollie rarely comes up as a request, so it's not there as a default.  

By the way, the current version is 3.6, and 3.7 is going to be out very soon... It has a ton of updates!  :) 

specifications:
20 simple products: name, description, image, price
mollie payment provider
translate webshop to dutch (language pack?)
template for pdf invoice (fully customizable)
templates for mailnotifications
dutch address format
custom fields for checkout (membernumber, etc)