Muqsiṭ is the active participle of aqsaṭa and means someone who acts justly. By contrast, qāsiṭ is used for someone who deviates from justice. The two similar forms thus carry different moral senses.
Afḍal is the comparative/superlative form; man yamshī is a relative construction. Their function is to heighten praise, not to record a measurable historical ranking.