i could not see some information around so what i have seen so far.
S > A > B > C > D
even if mission failes if you have collected the item it gets on your list.
yet i have not confirmed if the mission completes and you have not collected the item and if it still gets in your list. I hope someone can confirm this.
Right now i have only one S Rank and it's
Longdist attack power +16
Shortdist attack power +20
Btw i dont know what these numbers go for. i mean does +16 mean -16 hp? And also about other items there are other variables. What do they mean?