"Outstanding meritorious service" emphasizes that a person's contribution has a significant position in the nation, which means his outstanding and significant contribution to the country and the people and his special contribution. This is a commendatory term that most people can't describe.
"meritorious service" emphasizes that someone's contribution is beyond ordinary people and has excellent quality. It is usually used to describe people who have made outstanding contributions, emphasizing the height of personal ability and achievement.
Generally speaking, "outstanding meritorious service" emphasizes the particularity and importance of contribution, and "outstanding meritorious service" emphasizes the height of personal achievement.