While there's no "best cardio" for lean legs, combining aerobic exercise to burn calories and strengthening exercises for toning will help lean out thick legs.
My Fave Way to Build Strong, Lean Legs - Muscle & Fitness
Personal trainer reveals her tips and tricks for slimming your legs
How To Slim Down Muscular Thighs - Toned Legs Without Bulking
Thigh Exercises While Sitting - Thigh Workout
9 Common Physique Problems and How to Solve Them - Muscle & Fitness
How To Slim Down Muscular Thighs - Toned Legs Without Bulking
The Best Way To Train Legs
How to Reduce Thigh Fat, 7 Easy Ways to Reduce Thigh Fat
Images
How to Slim Muscular Legs - The Tricks and Science Behind Muscle
What Causes Thigh Fat and How Can I Lose It Naturally?