He is the _____ in the class

The correct word to complete the sentence is best. This is because the structure indicates a superlative degree is required to compare one student with all others in the class. Best is the superlative form of good and shows the highest level of quality or achievement. Other words like top prime or lead can describe status but do not fit as grammatical superlatives in this sentence. The use of superlative forms is a key part of English grammar and helps in expressing maximum degree of qualities. Thus the answer best is correct and fits perfectly in this context.