Your favourite being?

It's a toss-up between щ /ʃtʃ/ and ы /ɨ/. The former occurs in most Slavic languages, but also in Genoese Italian, e.g., scciêppâ /ʃtʃø'pa/ 'to explode' and the latter is not to be confused with /ɯ/ (the dot-less Turkish ı).

99.999...%

An ex-colleague of mine is a Slovak, and his wife was trying to teach me a little Slovak. The Ethnologue database puts Slovak speakers at just over 5 million. Out of a total of 6 billion people that would be 6000 to 5 that would be less than 8/100th of a percent. Unless I've most likely got my math wrong.

Ceci n'est pas un seing.