For P3D... you can't go past the FSLabs A32x series... especially if you want a "study level" aircraft. So many of the systems and FMGC components are modelled and functional.
And the ATSU stuff integrates so well with Simbrief and GSX, that it makes planning, configuring and setting up your flight really easy. Plus there are plenty of great tutorials on YouTube from the likes of Blackbox711 that show how the various features and functions work.
On the Boeing side, PMDG aircraft are good... and the QW787 is supposed to be OK.
I can't really comment on X-Plane or MSFS as I don't use either of those.