I think we'd all agree the JFK assassination is the most difficult. And out of a score of 10 in terms of difficulty, it gets a clear 10. A Warren Commission investigation which has a report and 26 volumes followed by a HSCA report with 12 volumes. Then you have the millions of documents with the ARRB.
How do people rate the RFK and MLK assassinations in comparison? I think the RFK ranks as a 2 in terms of difficulty with the MLK ranking at 4.
1 - JFK (10/10)
2 - MLK (8/10)
3 - RFK (7/10)
The first two have been investigated the most from local law enforcement to federal law enforcement to Congress. So understandably, those are the most complex assassination cases.
In the RFK case, there's pretty much only the LAPD and FBI files and little else to research. I don't think the HSCA dug deep into the RFK assassination.
There are Rabbit Holes in all three cases but I think the JFK case has the most weird coincidences (most of which are probably just coincidences).