Are barbell squats superior to all other forms of lower-body training (leg presses, hack squats)?

If the goal is primarily hypertrophy, then, NO - leg presses, hack squats, barbell front squats, Smith machine squats, sissy squats, or any similar compound movement can build muscle mass as well as barbell back squats. Any of those done progressively will stimulate muscle growth and will continue to stimulate growth until genetic limits for leg mass are achieved.

If the goal is powerlifting progress, then, YES, perhaps obviously, back squats are superior to the others.

If the goal is adding strength for Olympic lifting, YES, a case can be made for either barbell back squats or barbell front squats being superior to any other direct quad work

If the goal is primarily conditioning and/or general strengthening, for, say, athletics, YES, barbell back squats are superior since they allow for greater poundages to be more-simply handled while requiring a much-more integrated, coordinated effort of not only the legs but also the supporting muscle groups of the hips, back, and abdominals, than do front squats (which can be more difficult to perform because of needing to handle the bar above the chest), leg presses, hack squats, sissy squats, or Smith machine squats.