Kevin Conroy's most famous line as Batman is probably "I am vengeance. I am the night. I am Batman!" but I don't think it's his best. Rather, I think it's the scene from
Mask of the Phantasm where Bruce—not Batman, but Bruce—pleads for forgiveness at his parents' grave for being in love.
"I know I made a promise, but I didn't see this coming. I didn't count on being happy..."
Kevin Conroy is the only actor who has ever played the role who could convincingly play that scene. There's one little touch that is absolutely masterful and gets me every time: the way he delivers the word "happy". His lips were wet. It makes sense because the scene takes place outside in the rain, but the man delivered the line in a recording studio, so either he had the forethought to splash his face with water to simulate the rain... or tears really were streaming down his face.
Either way, I don't believe anyone else who's been Batman could do it.