Completing hip flexion above 90-degrees will minimise contribution from the rectus femoris (one of the quadriceps muscles that is also a weak hip flexor) to directly load the iliacus and iliopsoas muscles. General prescription: 3-5 sets, 5-15 reps each side.
This exercise will load the hip flexors in their shortened position, but will also condition the rectus femoris (one of the quadricep muscles, also a weak hip flexor) in its shortened position, to keep the knee straight as the hip flexes. General prescription: 3-5 sets, 5-15 reps each side.
This is by far my favourite hip flexor exercise. Using the cable improves the loading of the hip flexor through full range of motion, and laying on the ground improves hip and trunk stability to increase force production of the hip flexors. General prescription: 3-5 sets, 8-15 reps each side.
The hanging hip flexor raise is a great overall movement to train hip and trunk flexion at lengthened ranges. It will also improve grip strength (unless you want to bias the hip flexors fully and use ab straps) and general shoulder endurance. General prescription: 3-5 sets, 5-20 reps.
The stability of the machine and ability to drive the feet up to encourage hip flexion makes this hip flexor variation suitable for increasing volume and load, without risking compensation from other muscles or motions. Ensure endurance, capacity and technique is adequate before adding load mindlessly. General prescription: 3-5 sets, 5-20 reps.