Which promised girl is Raku referring to?
As the manga has ended, we have a winner. And the promised girl is

(click to enlarge picture, pictures taken from chapter 221)


   Chitoge





   


But wait, there is more


   Chitoge overheard that Raku actually like Onodera





   





   So Chitoge gave the key to Onodera





   


So, in the end, the promised girl is


   Onodera





    


So to answer your question, the girl with the key to the locket is the girl whom he promised to love