Soldato
The responsibility is 100% in Vettel. This was his year but because he is so generous, he gave away the title so easily. Mistakes in China, Azerbaijan, France, Germany, poor qualification driving in Hungary, then mistakes go on and on till the last race.
They didn't even fight for the victories in the last couple of races, when instead of going with aggressive tyre strategy, they always chose the hardest and slowest compound. What for?
Now, for the next year, it all depends on the Ferrari organisation as a whole to bring a good car for Leclerc and not to screw things up in the winter break.
Ferrari made a few mistakes themselves. About 4 I think