。
Congratulations on winning the debate contest.
Literal
Debate-contest [in-で] winning-(honorific) congratulations.
優勝されて uses ~される, one of the honorific (sonkeigo) patterns — derived from the passive form but functioning as an elevated 'do' for the listener. Equivalent to 優勝なさる but more modern-feeling. ~ておめでとうございます is the polite formula for 'congratulations on having done X.' Structurally, the te-form gives the reason, and おめでとう is the congratulatory adjective (from めでたい).