I know this may sound crazy, but if I was ever forced to choose just one knee exercise to prescribe, it would be self knee extensions with over-pressure in sitting.
I learnt this at a McKenzie course many years ago as part of the “Treat your own knee” concept.
If you have any knee pain, either from patella-femoral sensitivity or knee OA, try this. First do the movement that hurts, for example jog, try a step up or a deep squat and rate your pain. Next, sit at the edge of a chair, with the heel on the floor and push on the distal femur to force the knee into maximal extension with or without quads contraction.
Repeat 10-15 times and re-evaluate how you feel. If better in any way, then repeat this every 2 hours for 2 days.
Some of my patients have thought I was a genius for prescribing them this simple exercise. If however, repeated knee extensions worsen or make no change, then obviously pull out other weapons.
I explain to them that this exercise may feel good, because for most of the day our knees are flexed to 90° as we sit.