Smoothest surface...

in the solar system: Saturn moon's mirror-smooth lake 'good for skipping rocks'. Key bits:

"The largest lake on Saturn's moon Titan is as smooth as a mirror, varying in height by less than 3 millimetres [across a 235 km distance], a new study shows."

"Spectral data also showed that the apparent lakes seem to be filled with methane and ethane, which would be liquid on Titan's frigid surface. And "geomorphologically, they just look like lakes", Zebker says."

Here's the technical abstract.