There's no definitive list, people like different stuff, but if it helps you I'm using:
Ricardo's cockpit, not only it looks cool but I can also read the panels better.
Proper neck mod (great for trackir)
^Those are probably used by majority of pilots...
Bigger HUD font if you're blind like me, but don't overdo it or the letters will overlap too much, I'm using 62.
I also like GAU-8 dispersion mod, but I don't know if it's compatible with multiplayer.
Snoopy wpn repaint.
Eddie's CMS programs.
RAE mod is quite popular as well.