I'm going to start with the assumption that we've stopped terraforming Earth away from a place where we can live and gotten that situation under control.
The Bussard reactor has been perfected and they are now widely in operation providing most of Earth's energy needs; the balance being provided by clean renewable sources. Bussard reactors have supplied energy for accelerator based nuclear waste treatment that has transformed the nuclear waste from the fission plants into stable isotopes, it's provided the energy for large scale automation and recycling for many goods that previously just would have been too energy intensive to recycle. It's providing energy for biofactories that has removed carbon-dioxide from the air and returned oxygen. Many other areas have been cleaned up, agricultural animal waste is no longer allowed to enter streams, rivers, and oceans, neither is human waste, chemical fertilizers, or other things responsible for huge dead zones. The beaches are clean now, the water transparent.
The Bussard reactor has been sufficiently advanced that it is now small and light enough to be readily launched into space and used for space propulsion. If we could generate enough power to accelerate at a 1G rate half-way then decelerate at 1G the rest of the way, we should be able to get there in a couple of days. The idea of the Bussard fusion ramjet requiring a scoop of 100 miles isn't necessarily as infeasible as it seems, for the scoop need not be physical. Since most hydrogen nearby at least is ionized, it can be a magnetic scoop that steers the charged hydrogen atoms, rather than a physical scoop. If we could do this then transporting materials to Mars would be a relatively minor exercise.
Now once we are there we need to supply heat; ok, lots of Bussard reactors, to melt the carbon dioxide at the poles and also free up CO2 that may exist in the regolith. Most of the nitrogen of Mars is probably tied up as nitrates in the regolith but these are only stable at low temperatures. So as we heat things up we should thicken the atmosphere with CO2, which will result in additional warming by trapping solar energy, and then we need to introduce plants to convert CO2 to O2, while continuing to release nitrogen from nitrates.
My idea here is to terraform Mars in a relatively short time, not the centuries required by less ambitious schemes. Here, we build enough of these reactors to really add large amounts of energy; and we build plant factories, huge factories with sulfur vapor lighting to grow huge amounts of plants rapidly to convert that CO2 to O2. The rate then is only limited by how many and how fast we can build these fusion power plants.
This all of coarse assumes there isn't existing intelligent life that might get annoyed by our activities. This big hole found on Mars, more than 100 meters across, with no visible walls or floor
, makes me wonder.