The most difficult, but in long run the most rewarding was a Royal P-51D Mustang. This plane took about 5 years to build. I would reach a point where I was stumped, stow it in the attic and go on to another plane. At some point I would stumble on to a "oh thats how you do that" get it back down and eventually finish the project. There were several trips to the attic.
Gene