For me, it’s the good old 1×1 round plate. The lip on it is a rib when they’re stacked. It forms a cap on any stud. The antistud serves as a thruster port or a HUD in a pinch. They form buffer transitions in color blocking between larger parts. In transparent colors, they add clutch while being easier to remove. It fits into places where a 1×1 plate won’t. It can even serve as a rotational hinge point when sandwiched. 1×1 round plate is my Universal Element.
