Each type of assessment has its own unique value and serves a specific purpose in facilitating student learning and success. Personally, in an ideal setting assessment AS learning may be the most valuable for students as it allows self-directed, reflective, and independent learning but perhaps we are still far from achieving the benefits from this type of assessment. Our students are still not used to complete independence where they would have the liberty to set their own goals, monitor and reflect on their progress.
This leaves me to my next choice, Assessment FOR learning which can provide them with an ongoing feedback during the course of learning, helping them identify their weaknesses, allowing them to reinforce their strengths and permitting them to adjust their learning strategies accordingly, and still under the guidance and supervision of the teachers.