The Paris Metro. Reliable and functional.
I like knowing that I can move about the city, quickly and easily (most of the time). It serves its purpose.
But my favorite way to see Paris is to wander. Sometimes with a map, sometimes aimlessly, always with my camera in hand.
Because down there in the Metro tunnels you can't savor the sights and sounds of the markets or explore the quirky shops and little alleyways or ensconce yourself in a café for several hours while you take in the street life.
To me, Paris is best explored on foot. With little or no destination in mind. Just wandering.