Squats #1
Spend time getting your form down before you Jack up the weight though. Increase your weight gradually week over week until you find your ceiling. Honestly if you do only 1 exercise for your lower body it should be squats.
Week 1
3
3
3
Week 2
5
5
5
5
5
Week 3
10
8
6
Week 4
4
4
2
Week 5
5
3
1
Rinse and repeat.