Over the years, speaking to and watching a lot of guys and even some very powerful women lifters, I find that front squats and bench squats the most affected for mass and strength. With less stress on the lower back area. With bench squats, most find it an excellent way to learn proper squatting protocol, with more leg/hip and less lower back involvement. Athletes find better performance, from field events to football, with either of these two excellent lower body movements.
Would also include one legged squatting style exercises. As weighted step ups. one legged squats, lunges, etc., to improve squatting abilities themselves.
Body flexibility comes with each lift the more these exercises are worked.
Good Luck.