The satellite that attracts the most attention is Titan, the largest satellite of Saturn. It is the only satellite with a substantial atmosphere.
Most satellites have too little gravity to retain an atmosphere.
The atmosphere of Titan is composed of hydrogen and methane among other constituent gases. It has an atmospheric pressure comparable to Mars and is known to house many organic molecules. Titan attracts an unusual amount of attention because of the alleged presence of hydrocarbons on its surface. These hydrocarbons would produce a brownish substance similar to that made during experiments on the origin of life. Thus, Titan is probably the satellite most likely to support life of any kind.