L Suspects Light — The Ultimate Bluff
L announces on live television — through a decoy — that Kira killed the decoy, not the real L. Light responds exactly as Kira would have. L had engineered the entire situation just to confirm his first real lead. He had already won the first round before Light knew they were playing.
Why it lands: The first time the audience understands that L is Light's equal and the real game has begun.