I'm a huge fan of the body weight Pistol. It tests the strength, balance, mobility and determination of the athlete like no other squatting movement can. Mastering it can take a while but once you get it you are ready to start adding some repetitions to the process. I also really like what are called Chinese or Airborne Squats. This is a body weight squat with the leg bent and touching down behind the opposite heel. Not as hard as the Pistol but still a great challenge. Check out the following video of Master PCC Al Kavadlo doing what he calls a Shrimp Squat. This is a great variation of the Chinese Squat and something I'm going to need to try myself!