For me Its those pairs of symmetrical 2x3 wedge plates . I was organising my wedge plates yesterday to ensure most of them are paired up nicely, and I found out its these 2x3 wedge plates that are severely lacking. Since I MOC mostly LEGO Transformers , these plates appear everywhere (torsos, limbs, tailfins, chests and even as robot faceplates ).
Sadly the procurement of such parts from bricklink is tricky. many sellers offered a piece in one side ,and without the opposing ones in my preferred color. *duh*