I find this a hard question to answer as there is no case where a silent protagonist couldn't be improved by *not* being a silent protagonist.

As such 'characters' go, Joker is pretty decent. And Ludger from Xillia 2 has an incredibly interesting story, his is a really weird case where before the player takes control of him, he seems to live a rich, full life as an established character. Then when the player takes over he is a blank slate with no personality. Similar kind of thing with the player character from KOTOR; they have a really interesting story attached to them, but no real character.