There is no single best assessment tool or strategy. What works well in one situation may not work well in a different situation. There are too many variables that affect where, when, and how a particular assessment should be used. Those variables can be anything: the subject content, the number of students in a class, the age of the learners, the type of learners that are present, etc. This means that the "best" assessment strategy is the one that works best for a given situation and/or scenario. Formative assessments are great because they give students ongoing feedback on their learning throughout the learning process. Students are able to make adjustments as they go; however, that kind of assessment gets more and more difficult as a class gets larger. A teacher simply can't get to every student each period if the class is 38 students large and the period is 43 minutes long (personal experience). A summative assessment can have great value. It does a nice job of showing an educator how well a learner(s) has understood an entire body of content over an extended period of time. Unfortunately, summative assessments provide mainly hindsight. By the time the student takes the test/exam, there is very little opportunity for the teacher to make corrections to student learning. A teacher may have to go back and reteach content based on a class that did poorly on this kind of assessment.
Regardless of the type of assessment used, it is critical that the educator gives the assessment to the student sooner rather than later. More immediate feedback allows the student to make learning and understanding adjustments while the content is still fresh. Written feedback is a solid way of giving feedback to a student because that student can examine the feedback multiple times; however, verbal feedback is sometimes better because it is a quicker and more efficient way of giving the feedback during a period.