What is the best hamstring stretch?

There is a lot of good ones, but try this one. While seated on the ground with your legs straight out in front of you place your left leg on top of your right leg and relax it there. Flex your right foot straight up with your foot flat. Put your left hand on top of your right hand reach towards that right foot. Drop your head and relax. Really pull that right toe back, so you get a calf stretch as well. Press your lower back towards your knees. Hold for 15 seconds and switch legs.
Active Isolated Stretching (AIS)

and Self Myofascial Release are my go to stretches for the hamstrings. To stretch the hamstring using AIS, simply lay on your back with both legs extended. Take a towel or small rope and wrap around the ball of your foot and use as assistance when actively moving one leg to the sky/towards your head. When you feel general tightness, hold for two counts..return and repeat. I suggest 10 reps with each leg to start.
I've included a video for Self Myofascial Release (foam roll) as another option.
I'd also suggest you stretch everything from the ground up to ensure your tight hammies are not a result of an issue elsewhere. 
Although I wouldn’t say there is one stretch that is the absolute best for your hamstrings, a combination of different stretching types and methods would be beneficial. Self myofascial release (foam rolling or self massage), static stretching (holding a stretch for an extended period of time), and active-isolated stretching (holding and releasing a stretch every 2 seconds) are a great place to start.

A great static stretch for your hamstrings after a workout is completed and your muscles are warm, is to lay on your back. While keeping one leg on the floor, bring the opposite leg straight up as far as you can while keeping your hips flat on the ground. This can be accomplished by wrapping a beach towel around your foot while holding the ends with your hands to control the tension of the stretch.

Hamstrings are a common muscle group that is tight on many people, but be sure to also stretch other muscle groups as well to help counteract any muscle imbalances you may have.

Mark Levine
The main idea of stretching is making sure you incorporate a variety of stretches. Before you exercise, I recommend dynamic stretches, such as squat jumps. I would follow that up with self myofacial release(SMR) or foam rolling. Put the foam roller on the floor and sit on foam roller. Proceed to foam roll your hamstrings. When there is a knot or soreness, hold it for 15-20 seconds. Then proceed to finish rolling your hamstrings. After your workout, do some static stretching. Start with your legs together with a little bend in your knees. Proceed to touch the ground or go as low as you can. Hold for 10-15 seconds. Do 10-12 reps.

In a standing position, with an exhalation, step or lightly jump your feet 3½ to 4 feet apart. Rest your hands on your hips. Turn your left foot in 45 to 60 degrees to the right and your right foot out to the right 90 degrees. Align the right heel with the left heel. Firm your thighs and turn your right thigh outward, so that the center of the right knee cap is in line with the center of the right ankle.

Exhale and rotate your torso to the right, squaring the front of your pelvis as much as possible with the front edge of your mat. As the left hip point turns forward, press the head of the left femur back to ground the back heel. Press your outer thighs inward, as if squeezing a block between your thighs. Firm your scapulas against your back torso, lengthen your coccyx toward the floor, and arch your upper torso back slightly.

With another exhalation, lean the torso forward from the groins over the right leg. Stop when the torso is parallel to the floor. Press your fingertips to the floor on either side of the right foot. If it isn’t possible for you to touch the floor, support your hands on a pair of blocks or the seat of a folding chair. Press the thighs back and lengthen the torso forward, lifting through the top of the sternum.

In this pose the front-leg hip tends to lift up toward the shoulder and swing out to the side, which shortens the front-leg side. Be sure to soften the front-leg hip toward the earth and away from the same-side shoulder while you continue squeezing the outer thighs. Press the base of the big toe and the inner heel of the front foot firmly into the floor, then lift the inner groin of the front leg deep into the pelvis.

Hold your torso and head parallel to the floor for a few breaths. Then, if you have the flexibility, bring the front torso closer to the top of the thigh, but don’t round forward from the waist to do this. Eventually the long front torso will rest down on the thigh. Hold your maximum position for 15 to 30 seconds, then come up with an inhalation by pressing actively through the back heel and dragging the coccyx first down and then into the pelvis. Then go to the left side.

One of the best hamstring stretches you can do is the 90/90 stretch. It requires you to lay on your back and extend the leg you’d like to stretch, while holding the thigh with your hands. If you’re having a hard time gaining flexibility in your hamstrings, you should consider laying on a foam roll before doing the stretch.

By laying on the foam roll, you signal the tissue surrounding the muscle and the muscle rolled to relax and let go. The muscle will be much more “willing” to lengthen after it’s been inhibited by foam rolling!

Place the foam roll on the spot that feels most tender in your hamstring for at least 30 seconds, and directly follow it with a static stretch of at least 30 seconds. In this case, the 90/90 stretch is perfect!

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.