A metallic astroid near the systems frost line
Defining "easy" as the shortest time until it is habitable with the smallest resource investment, planets are all together a bad idea. What is a much quicker, safer and more efficient solution are large habitable space stations using the centrifugal force to stimulate gravity.
Using steel or silumin (aluminium-silizium alloy), you get radi of up to 25 km which means you can get livable areas the between the size of the Brunei or Cyprus and up to Panama or Austria. Using carbon based nano materials, radi of roughly 1000km become possible. Those materials get you the land area of India or Australia on the low end and slightly less than the land area of Earth or the surface of Mars on the high end. on the high end. This assumes you go for small radi and 5x the diameter in length on the low end or for the maximum radi and 10x diameter in length on the high end. Of cause, there is no real length limit. You can have a tube of those looping around the local star as many times as you want. This is then called a Topopolis.
Furthermore, those constructs like coming in pairs, as that helps them with gyroscopic stability. Either add a second cylinder inside or connect the second one using an external superstructure. All it has to do us rotate counterclockwise.
Choice of location those stations can be built wherever you want/need them. As your fust step in settling a solar system should be the establishment of space based industry anyways, those habitats benefit from the low launch cost. Just get an icy, rocky, carboniferous and metallic astroid and equip them with mass drivers, laser launchers or launch tethers. Then, set up solar power satellites in low solar orbits, which beam you the energy via laser and your industry is set.
Build to your preferences. The habitats are vastly more flexible when it comes to the environments you can build in them. This goes for environmental and social systems. Smaller, more independent habitats can allow more flexible designs and reduce interdependence. If groups are discontent with the way a state is run, they can get their own country.
Planetary protection. You don't antagonize conservationist groups doing this. Just leave the natives, be they bacteria or blue skinned native American stand-ins alone.
Safety. Planets are dangerous. Earthquakes, Tsunamis, Volcanos, Hurricanes, Blizzards, ... What sort of space ork would want to live on a thin layer of rock above a hellish magma-pit? And that's not even considering how fast and easy pathogens, nanotechnology weapons or missiles can be sent from one polity to another. And that's not to mention the radiation issue. Radioactive materials are just strewn about in the environment, only a magnetic field protects from particle radiation and the thing gas layer above you is just transparent to some parts of the UV spectrum. Inhabiting planets is sheer madness!
Ressource efficiency. Per square meter of habitat area you need a few tons of material. A planet is using orders of magnitude more mass per livable area. This is just a waste of resources.
Scalability. A full on habitat smarm can use the entire energy of its star. Terraforming is, even one one uses Birch Worlds to terraform gas giants, stars and black holes, a dead end in that regard. If you want a planet, you can just construct it using hydrogen and a shell. That is the only way your resource efficiency could come close.