Guild Wars 2

Does the world completion have to all be done on one character, or can it be divided among characters? E.g. if I explore all of Metrica Province as one character and all of Caledon Forest as a different character, will my % in the character select screen reflect both zones being completed? Or does it just show the highest % that an individual character has done?

All on one character I think.

All on one character. You can get world completion multiple times across multiple characters.

Like you said, the % in the char select screen shows the highest completion % done by one of your toons.

All on one. You need to do it multiple times for more gifts of exploration for legendaries.

100% map completion is all on one character. Getting the various exploration achievements (like Shiverpeaks, Orr, etc) is from all characters. To get those achievements every zone of a region must be visited by one or more of your characters. There are some zones that do not contain a WP, POI, Vista, SP or Heart so you may not visit them by doing the 100% map completion.