Given that they are finding water in all sorts of places, my bet is on Enceladus and Mars.
But here's a few interesting quotes I gathered from NASA...
From Water: Life's Elixir in the Solar System:
Liquid water is a necessity for every form of life known, with the
possible exception of some plants or fungi that may get by on water
vapor. With this in mind, scientists are eagerly searching for liquid
water in places other than Earth.
In recent years, NASA spacecraft managed by JPL have found a
tantalizing sprinkle of clues supporting both the possibility that
liquid water may persist below the dry surface of Mars and the icy
surfaces of three large moons circling Jupiter. With NASA's strategy
to "follow the water" in the search for life, Mars and Jupiter's moon
Europa are priority targets for future robotic missions.
From The Solar System and Beyond is Awash in Water:
There are several worlds thought to possess liquid water beneath their
surfaces, and many more that have water in the form of ice or vapor.
Water is found in primitive bodies like comets and asteroids, and
dwarf planets like Ceres. The atmospheres and interiors of the four
giant planets -- Jupiter, Saturn, Uranus and Neptune -- are thought to
contain enormous quantities of the wet stuff, and their moons and
rings have substantial water ice.
Perhaps the most surprising water worlds are the five icy moons of
Jupiter and Saturn that show strong evidence of oceans beneath their
surfaces: Ganymede, Europa and Callisto at Jupiter, and Enceladus and
Titan at Saturn.
Scientists using NASA's Hubble Space Telescope recently provided
powerful evidence that Ganymede has a saltwater, sub-surface ocean,
likely sandwiched between two layers of ice.
Europa and Enceladus are thought to have an ocean of liquid water
beneath their surface in contact with mineral-rich rock, and may have
the three ingredients needed for life as we know it: liquid water,
essential chemical elements for biological processes, and sources of
energy that could be used by living things. NASA's Cassini mission has
revealed Enceladus as an active world of icy geysers. Recent research
suggests it may have hydrothermal activity on its ocean floor, an
environment potentially suitable for living organisms.