Logically I have to think it is Assange as he is very high profile but moreover he has seriously damning material on the people who have always been going after Trump and rotten journalists as well. I always wondered why Britain was so fast to swoop in and snag Assange before the US got him as the data we always thought he had would have decimated the DNC on so many levels. But now that we heard about extradition to the US for Assange, the immediate data dump by WL on Kamala Harris after she was picked for VP and the often discussed dead mans switch has to put Assange right up there as most likely choice.
I could definitely see a deal be made with Assange to turn over all materials and get a pardon. We have to consider admissible evidence as if that data comes from someone classified as a journalist it can be used but it can't be used if obtained through spying means or, better stated, it can easily be challenged that way. Through Assange though it would not go through that ringer and it would allow all the corrupt journalists and everybody else involved in anything improper that was reported to WL to be scooped up.