Any back run I am working on is the hardest. Everything else is easy!
I think the E92 & friends can be real hard bc, as DJ mentioned, you have Nadja, and blank, and blue/black/maybe red crofts, T216s that could include all 3 Kotton back variations, MINO, and VE (which is as hard as anything), Tango eggs, Mothers bread, etc. I mean Crofts Cocoa is cake when compared to some of these, and forget the E105, E106, E101, E102, Dockman, and whatever else I forgot.
Here is a link to my Wagner throwing and bat off back runs, still very much works in progress. And I am softly attempting the same folly with Cobb on bat, but as DJ mentioned, that is nuts.
https://www.flickr.com/photos/156271...57703230513722
Another very underated back run would be the Joe Jackson M101-4/5. Altoona, Homes to Holmes, Successful Farming, etc. That would be crazy hard.
And, yes, the Super 6 in T206 would be rough -- I am almost done with Evers Yellow Sky (eslewhere in that link).
Of course, a T206 Wagner or Plank run would be awesome. Not many in the run, but not many period.