A "crowning achievement" is a great success worthy of much praise and respect.
Example: In video games, a "jobs" system allows role-playing game characters to learn different skills suitable for different "jobs" (role-playing professions), such as knight, wizard, priest, thief, and so forth. While this type of role-playing game feature truly began with the Dragon Warrior series (Dragon Quest in Japan), this was adopted, and greatly expanded upon, by the "Final Fantasy" series of video games. Popular with gamers, the "jobs system" is often considered the crowning achievement of the series, giving players a deep personal connection to their fictional alter-egos.
This is to say, it is considered a great success worthy of giving great praise.
A crown is not only a physical treasure; "the crown" of something is its peak, its highest point, its pinnacle. (i.e. "the crown of the head," "the crown of a tooth") These words are easily used as metaphors for success.
(This post is based on the first paragraph from http://www.1up.com/do/previewPage?cId=3181309 .)