I've seen a lot of transit logos all over the world, and this is my personal favorite.
Perhaps one has to know Paris to appreciate it. But that's fine; it's a logo for Parisians.
In Paris, you need only draw a circle with a curving blue line through it, and anyone will see that as the city and its river, the Seine. Huge liberties can then be taken with the shape of the Seine (map version here) to suggest the outline of a human profile. That the face seems to be looking upward is brilliant; it conveys reverence and relaxation in the same moment. To take the city's original life force, the river, and render it as a face can take you deeper into the idea that Paris itself is a being, a consciousness, not just a playground for our egos.
(UPDATE: If you go to the RATP website, you'll see the slogan that goes with the logo: Aimer la ville. Literally, "Love the city." )
I also love the fact that it's not trying to be about transport in any literal sense. It aims higher. Transport in a big city is like the circulation of blood. It doesn't just enable urban life, it is urban life. It's not just a product or a service, it's a way of describing what the city is.
For whatever reason, I feel warmth and wisdom from this little logo whenever I see it.