There are no exercises that are "best for definition". As I point out in one of my works, definition comes only from diet. Since I believe you may be speaking of tone (the quality of the muscle's appearance when flexed), there are only a few exercises.
While the barbell squat and leg press are best for gaining mass, the leg extension and standing leg curl are your most "contracted" movements. As this is the case, finishing the workout with these exercises would be best. Holding to repetition at it's uppermost point (the fully contracted point) will aid in defining the "muscle bellies". The sissy squat can be a good shaping move, too. Again, flexing AND squeezing the glutes will give you the best results.
The use of static holds or static contraction is extremely helpful, too. This would be the practice of holding to contracted phase 'til complete exhaustion.