The basic requirements for human life is: a Nitrogen-Oxygen atmosphere and water. Most rocky planets will have all the other raw materials, which can be mined. In our star system, there are two planets that are potential terraforming candidates: Mars and Venus.
The atmosphere of Venus is 90 times more massive than Earth and nearly all of it is Carbon Dioxide. While Nitrogen only makes up a small percentage of the Venusian atmosphere, there is still four times more Nitrogen than Earth. One idea I read about was to build a sunshade to block all the sunlight and allow the atmosphere to turn into Carbon Dioxide snow, then bury it. This is probably the fastest method but would still require massive amounts of energy to bury a frozen Carbon Dioxide ocean covering over half the planet. Once it’s buried, you open up the sunshade and let just the right amount of light through to create a temperate climate. Another option is to convert the Carbon Dioxide into water oceans but there is no Hydrogen on Venus so it would have to be imported. The closest sources are Earth, Ceres and then the ice moons. Venus is almost the same mass as Earth, so people would weigh about the same. Venus is closer to the Sun, so it has to deal with more radiation but that can be solved with a sunshade. Another problem with Venus is that a day is almost as long as four Earth months. Even if we converted all the CO2 into water oceans, there may still be some super-rotation, which would equalize the heat between the day and night sides. You could also use a sunshade with shutters on the day side and mirrors to bounce light around to the night side, to create artificial day-night cycles on both sides of the planet. Speeding up the rotation would require massive amounts of energy, more energy than all the water in the star system being converted into liquid hydrogen and liquid oxygen for rockets. One approach I read about that can gradually change the rotation of Venus is similar to a space fountain but the projectiles orbit the Sun and are accelerated using solar powered satellites. When the projectiles reach Venus, they enter a tube along the equator and some of their momentum is transferred to the rotation of the planet. This approach would require freezing all the CO2 out of the atmosphere and even then, there is still four times more Nitrogen than Earth. It would experience losses due to atmospheric friction and this method would take tens of thousands of years or more to create an Earth-like day-night cycle. I also tried calculating whether you could use super-rotation energy to spin the planet but it would take too long.
Mars has a small amount of water, a thin Carbon Dioxide atmosphere and a small amount of Nitrogen. It may be enough for pressurized habitats but for a full scale terraforming, Mars will need imported Hydrogen to make water and more Nitrogen for the atmosphere. Mars is much smaller than Earth so a person would weigh less than half of what they would weigh on Earth. Since Mars has a smaller mass, it will require an atmosphere with more mass than Earth in order to create the same amount of pressure that Earth has. The closest sources of Hydrogen are Earth and Ceres and then the ice moons. Earth has a larger gravity well than Ceres, so Ceres will require less energy. The closest sources of Nitrogen are Earth, Venus and the moon Titan. Mars has no magnetic field so the atmosphere would be gradually stripped away by solar wind. This could be solved with an EM shield at the Lagrange point.
My approach is more brute force. I first build a Star Tram and what I call an Autonomous Self-Replicating Industry. The ASRI is made up surveyors, excavators, haulers, loaders, feeders, crushers, separators, smelters, mold casters, part casters, grinders, 3d printers, assemblers and molecular manufacturers. The machines individually are not self-replicating but they as a whole can self-replicate. Fractal Federation uses the Star Tram to launch these machines towards Luna, Mars, Venus and the asteroid belt. The ASRI is powered by Thorium reactors which should be available but I’m still waiting for NASA to do some surveying. Once they arrive at their destinations, they self-replicate for ten years to increase their industrial capacity. Once self-replication is completed, the ASRI builds a sunshade for Venus, an EM shield for Mars, aerostats to mine Deuterium and Helium-3 from the atmosphere of Uranus, fusion-powered factories on the ice moons to mass produce Hydrogen and Oxygen, landing pads on both Venus and Mars, Nitrogen extractors and storage tanks on Venus, and 3.5 billion 1 gigaliter supertankers propelled by hydrogen rockets. The supertankers move Hydrogen and Oxygen to both Venus and Mars and Nitrogen from Venus to Mars. Some of the CO2 is converted into Oxygen and the rest is combined with Hydrogen to make water oceans. There is enough supertankers landing and taking off, that the water created in their exhaust is a significant contribution to creating oceans. My plan could complete terraforming by the year 2600. It could be quicker or longer depending on traffic jams.
Here is the spreadsheet I made to calculate terraforming. I still find mistakes and I haven’t found anyone willing to check it for me. I think I’m in the ballpark which is good enough for my purposes.