Nothing will never be better than heavy barbell squats, but if variation is needed there's also (in this order) split squats and front squats to try. The only

Nothing will never be better than heavy barbell squats, but if variation is needed there's also (in this order) split squats and front squats to try. The only exception is low-bar/leg disability, then I think it might be better to keep it bodyweight and machine oriented (such as leg press).