There is a site, Terraforming Wiki. Even if it is not written by scientists or supervised by an institution, it still contains some useful information about terraforming and very interesting points of view.
There are many things to take into consideration when it comes to terraforming. Here, I want to point out only 4 of them.
- Gravity is very important. A too low gravity will make a celestial body lose its atmosphere. If one day we create an atmosphere around the Moon, it will at some point be lost in space, even if that will take more then a human lifetime. This takes out of consideration smaller moons and dwarf planets.
- Water is the second most important. All inner planets and moons (except Earth) have a lack of water. If we don't bring water to them, they will become large deserts. By contrast, moons of the outer planets (except Io), if heated, will become ocean worlds. Also, other volatiles are missing where water is absent.
- Luminosity is the third constrain. Plants need a certain amount of light and also they need both red and blue light waves. I made simple experiments with plants (see here). Beyond the asteroid belt, plants still can grow, but I don't know if they can feed a human colony. Beyond Neptune, plant life is probably impossible.
- Chemistry of the target planet or satellite is also important. Some chemical compounds might be dangerous for life. Some substances can be transformed, others cannot. Venus has an atmosphere rich in carbon dioxide and sulfuric acid, but both compounds can be transformed. By contrast, Titan might have an internal ocean as salty as the Dead Sea based on Cassini findings. We still don't know how to desalinize a planetary-sized object.
If we look at these four constrictions, we can say that no celestial body is easy to terraform. Also, there are greenhouse gasses that can rise temperatures even for the distant Pluto to values suitable for life.
Unfortunately, there are many things we don't know. For example, we still don't know what is the amount of water available on Mars. Will it be enough to create an ocean? On the other hand, many of the moons of the gas giants seem to have salty planetary oceans, which might not be suitable for Earth life. We know that Enceladus has an alkaline ocean with a pH of 11 to 12 and that also Europa has alkaline salts dissolved in its ocean.
In conclusion, I don't want to point a specified planet or moon as the best candidate for terraforming. Only two things are to be said: that terraforming is very hard and expensive (much beyond our current technology) and that we need much more research data before pointing to a celestial body or another.