I would have to say it is my large plastic thread case--given to me by my hubby and oldest (only son at the time) for Christmas way back in 1978. It held the large floss cards--loved that size. At the time it held all my floss. Since then the floss cards have been downsized and I have added several small plastic cases for the over flow. I really hated that they changed the size of the cards. The smaller cards are so bulky with a full skein. It does have a lid but the hinges gave up a long time ago.
My family has given me gift certificates over the years and an occasional pattern. They don't have a clue as to what I use besides needles and thread! LOL