I meant the best format to keep clips in after editing. I'm collecting bits and pieces of edited video and stills that I will eventually use to make a movie.
Then a possibility is to keep it in the format that you plan to deliver your final in - because then you can use the Smart Render feature and you have no quatity loss from the final recompressing.
Of course under the condition that the final format is well editable.
Here I mostly deliver in MPEG-2 720p and do all the Deshaking, Neat'ing, Proxying etc. to that format before the real editing begins.