Definitely squats!!! I could never get definition in my thighs before I started squatting. I was worried that my legs would look "big," but they don't at all. I do 5x5 with heavier weight. I also do lunges, SLDL (for hamstrings), leg presses and hamstring curls. I've never had such fast results with my thighs before. Squats were definitely the key to this equation. Unfortunately, I've had to lay off squatting for awhile. In their place I have been doing heavy leg presses as my 5x5. I miss squats and can't wait to get back to them.