Average-sized planets and moons are the best destinations for terraforming. How should such a planet look like, depends on average temperature. Please note that the list is made of what a body with Earth size would look, in reality details would be far more complicated and much different from what is listed here.
Hydrogen gas Earths Edit
This is how an Earth-size planet would look when temperature drops to 15 K. There would be a tenuous atmosphere made of hydrogen and helium (even if hydrogen would rather sublimate and condense on the ground). All other gasses would be frozen on the surface. For Solar System, such a planet would be located in the outer Oort cloud (or it could be a Rough planet). It will have all elements needed for life, however.
Terraforming will request adding a source of heat and a source of light. Light from its hosting star should be too dim for plants to survive and greenhouse gasses might be in a toxic dose.
Methane sublimating Earths Edit
In the inner Kuiper belt, temperatures allow only a tenuous atmosphere to exist. Methane and a few other gasses (like carbon monoxide and nitrogen) can sublimate, but not in an enough dose to create a thick atmosphere.
Terraforming will require greenhouse gasses, to increase temperature. However, plants will still have not enough light to survive, so that they will need an extra source of light.
Note that this type of planet may be much smaller and lighter than Earth: the low temperatures allow much smaller bodies to hold an atmosphere of mid-molecular mass volatiles. If these bodies are heated to Earth temperatures, the volatiles will escape into space. Larger planets of this type will tend to have a massive atmosphere where methane ices coexist with hydrogen and helium; if heated to Earth temperatures, they will release the light gases and hold methane.
Methane - organic Earths Edit
The best example is Saturn's moon, Titan. Organic compounds are found as gasses, solids or liquids. Some nitrogen can be found in the atmosphere. Other gasses are frozen. The planet might have water and carbon dioxide, but they are solid.
If the planet is orbiting a Sun-like star, terraforming is possible by using greenhouse gasses. Once temperature is set, plants can be inserted. However, size and mass issues similar to the previous type exist with these planets.
Tenuous oxygen Earths Edit
On the orbit of Jupiter, oxygen is in gaseous phase. However, if the mass is high enough, there should be also other gasses in a greater percent, like methane or ammonia. They can interact with oxygen (they can burn when a lightning or a meteorite strikes). Another hypothetical planet would be entirely covered with ice, with an atmosphere made of oxygen, produced by ultraviolet and X - rays bombarding the planet.
For this kind of planet, all terraforming models proposed for Jovian moons can be used.
Cold carbon dioxide Earths Edit
If an Earth-like planet is located between Mars and Jupiter, this is what it is expected to happen. Temperatures are too hot for methane, but still low for carbon dioxide. The planet will have a tenuous atmosphere consisting of carbon dioxide that will sublimate in summer and condense in winter. It would be similar to what Mars looks like today.
Terraforming can use models listed for Mars. Those are probably the most easy planets to be colonized.
Liquid water Earths Edit
Earth actually is a lucky planet. It could have ended as a dry desert like Mars or as a runaway greenhouse planet like Venus. The thin line is difficult to define. There could be Mars-like planets on a hotter orbit then Earth, but also there could be Venus-like planets colder then Earth.
If one day a planet with liquid surface water is to be discovered, terraforming should be done very careful, to avoid any unwanted turns. The new planet will look like a new Earth. Its rivers would have made a far more similar Geography then what other terraformed planets would look like.
Greenhouse Earths Edit
The best model is Venus. At higher temperatures, a runaway greenhouse effect can start. In theory, there are two models of Venus-like planets. One of them, protected by a magnetic shield, could still have water trapped in its atmosphere. The other, would have other gasses.
Terraforming such worlds is difficult, but not impossible. First, a shield needs to be made, in order to keep heat away. This shield might be composed of mirrors or lens floating in space, balloons or some special gasses that reflect light in day and become radiation-permeable in night. Second step, settlers must improve the atmosphere and create water (if it isn't there). Water can be generated from sulfuric acid (leaving sulfur dioxide behind, to be further broken into sulfur and oxygen. Some models are shown on Venus.
Waterless failed greenhouses Edit
Right now, many scientists theoretize that water was the catalyst for transformation of Venus into a greenhouse. Water vapor produces a very strong greenhouse effect, which prevents trapping carbon dioxide into carbonate rock and thermically decomposes any carbonates. Later, solar winds decomposed all water vapor on Venus, leaving it dry with a thick carbon dioxide atmosphere.
If a planet is similar to Venus but does not have any water in the first place (or has very little), there is nothing to kick-start the greenhouse effect. Such world will be a hot desert with a normal atmospheric density, and thus it will be terraformable with relative ease (however, it will remain a single-biome desert like the fictional planets Dune and Tatooine unless massive amounts of water are introduced).
Tenuous atmosphere heated Earths Edit
The equivalent in Solar System is Mercury. Because temperature is too high and solar wind too strong, these planets have only a tenuous atmosphere. They have almost no volatiles, only limited deposits of ice at the poles.
Terraforming such bodies must be done in a similar way described for Mercury.
Molten lava Earths Edit
If a planet is in torch orbit around its star, temperatures exceed 1500 K. At that values, rocks start to sublimate and create a strange atmosphere around. The planet might be covered by a solid layer or it might be an ocean of lava. it may have many volcanoes due to tidal forces.
Terraforming this kind of planet is impossible if you don't move it in a further orbit. However, if there still is a solid crust, Paraterraforming can be an option. At close distances, planets should be tidal locked, so that one side will be experiencing huge temperatures, while the other side should be very cold. Alpha Centauri Bb is a good example for this category.
Unconventional Earths Edit
Despite this classification, there are many other possible models.
Moved planets Edit
What if Mercury will one day suffer a gravitational pull and will be thrown into interstellar environment? It will face freezing temperatures, but it will not have ices. On the other hand, what if one day Pluto will experience a gravitational pull and will come where Mercury is now? If will transform into a giant comet until its volatiles will be exhausted. Terraforming them will be a great challenge.
Carbon planets Edit
Imagine a planet made of carbon compounds. Titan is a good example. But if you take Titan into Earth orbit, increase its size and add other more heavy compounds, you will get a planet with rivers of petrol. Since its core contains a lot of carbon, graphite should be a common rock and diamonds should be found easily.
Terraforming a carbon planet is possible, if you bring water from comets and you create an oxygen layer, but with that much carbon, it might ignite anytime. Most likely, settlers would like Paraterraforming, in order to preserve the organic resources and to start mining diamonds.
Water planets Edit
A water planet should have a global ocean. We have colder models, like Europa or Ganymede. Terraforming them should be an easy task. Just add bacteria, algae and other plankton and all will go by itself. The problem remains that humans will have to use floating continents or at least floating cities.
Spoiled planets Edit
These kind of worlds are expected to be found around White dwarfs, Neutron stars or Black holes. They are created from the debris left from a supernova. Hydrogen is expected to have been fused completely. Without any water, these dry worlds would contain heavier elements. Terraforming would prove almost impossible, but they could be a paradise for miners.
Another kind of spoiled planets are those exposed to solar winds or strong radiations (for example, planets around B - type stars). They look more similar to Luna and could be terraformed in a similar way.
There might be many other types of planets, future settlers will find them and will look for the best way to make them a new, better home.