Good question! I have to think...
The bag I never used is the vintage mini Bolide in rouge vif. I like it but never used it yet maybe because I am not fond of the bolide. I once wanted to take orange bolide 31cm on travelling trip with me since it has the zip top but I changed my mind because it doesn't have any pocket inside the bag. So I took the birkin 35 instead.
Many bags I regert buying were green massai and some shoulder bags that I forgot the names, because they are too sporty for me. Also the picotin in blue jeans, the color is nice but too sweet for my style. So I sold them all. But I still have the picture of me carrying the picotin here. I only used it once.