Any assessment of the ‘best’ exercise for a purpose intrinsically contains some level of subjectivity, but your two criteria shorten the list of of exercises that we might consider. Body-weighted exercises, by definition, limit our working load, and thereby present challenges to their physical difficulty and the degree to which they can stimulate hypertrophy.
The difficulty of body-weight exercises tends to be governed, therefore, by their technicality, or by the loads that they place on small muscles. And both of those factors disqualify them from being considered in this assessment.
The second problem, of course, is that exercises are functionally distinct, and that all exercises therefore tend to stress one functional ‘set’ of muscles, leaving others only to assist, control, or stabilise. That is, if we are pulling, we are not pushing; if we are lifting our body weight with our torso, we are neglecting our legs.
Yes, there are certainly gross compound exercises that incorporate the majority of the body in some way, but our body weight is unavoidably distributed amongst them.
Thus, your criteria tend to favour a multi-phase exercise, and/or an exercise that inherently stresses a large functional chain of muscles. And since there is, as mentioned, a subjective element here, we might therefore consider either:
- muscle-ups, or
- one-legged squats.
Muscle-ups are a multi-phase exercise that contains elements of push and pull, and further incorporate the core and lower body for stability, balance, and inertia. And one-legged squats stress the whole leg chain, whilst requiring significant activation of the core in order to counteract the imbalance of lateral loading at the hip.
I hope that helps.