In my physics class, it is hard to point to one or two cross cutting concepts as most important , but the most obviously applied are probably : Energy and Matter: Flows, Cycles, and Conservation and Cause and Effect: Mechanism and Explanation. From conservation of energy to conservation of momentum, we discuss the energy in a system and the idea that total energy theoretically stays constant, even as the type changes. What forces cause the change is always being touched on.
Having the students reflect on the 7 CCC's and explaining where and how they apply is a necessary part of training them to think like scientists and ask good questions.