I mean, from the point of view of good quality software engineering. Apart from glib/gtk/cairo, the next best one I know of is Guile, but this is really just prejudice because I was active (in a very minor way) in its development 15 years ago. At that time there were several rather good people working on it, and the quality of the work they did is still evident today.
Are there any other 100% projects that are better? Needless to say, the only answers that are likely to be interesting to others will come with good, clearly stated reasons. But I would be happy to get a few responses like: look at GNU xxx because .... and a sentence about what it is that gives this impression.
Ian