Manstein's revenge
I'll elaborate a bit though.
I think a/c are much more difficult, as far as the build goes. Got seams to fill and sand all around the thing, clear parts to NOT screw up. For finishing, you can't just throw some mud on it to hide flaws. There is something to be said for a well done tank though. I know I couldn't do anywhere near as good a job on them as I have seen here!
My opinion of hardest to easiest: (just generaly speaking)
Scratchbuilt
Ships (well, some ships anyway)
Aircraft
Armor
Sci-Fi (kinda ties in with a/c and armor, depending on subject)
Whatever else
Cars, way down on the list