Dont't know if this thread is dead/closed or if the team has already decided on a final list, but here goes nothing :)
My suggestion is the now known as "Cromeleque dos Almendres" situated on a hill near the city of Évora, in Portugal. It's the largest megalithic monument in all the Iberian Peninsula and one of the most important in all of Europe.
I'm sending this pics along with the link to the IGESPAR (institute for the management of the arquitectonic and archeologic patrimony) where you can find some more info. If you need some help translating from the portuguese, just give me a heads up.