une atmosphère des plus romantiques ou une atmosphère de plus romantique

Question

Why is the superlative in the plural, when the subject is singular?

Accepted Answer

Hi Ian, 
No, it is always in the plural as you are comparing it with others:

une ambiance des plus chaleureuses = one of the warmest ambiances 
un gratin des plus succulents = one of the most succulent gratins ( bakes) 
un portrait des moins flatteurs = one of the least flattering portraits

I am not 100% about the English translation so if anybody wants to comment I'll happily change it.
Bonne Continuation

Answer

I think what i.l was asking was confusion between l atmosphere la plus romantique (the most romantic atmosphere) and here, which is the atmosphere the most romantic of all the atmospheres.  I had the same confusion until I read the question and answer so thanks