I think, and it's probably just a pipe dream, that before you consider the huge technical challenge to reforming or terraforming any of the planets where they sit today in orbit, instead find a technological way to move them closer to the Earth's orbit.

The tremendous costs of sending a very small probe to even our closest planetary neighbor Venus, pales in comparison with some sort of craft that can move huge amounts of atmosphere or water. I say if you're going to make that expenditure, spend it instead on trying to change planetary orbits so they're more accessible first so all our terraforming efforts are a bit simpler. Plus they'll all be getting the same relative sunshine as we are and I'd think efforts to make them more hospitable would be thus easier.

Has to be a way to put a huge energy source on a planet that flings stuff off the surface and imparts movement to it or the other possibity of slamming comets and asteroids into it. The second method also gives scientists the ability to excavate huge areas and do some serious geology as the work is in progress.

Now just imagine Venus or Mars on our same planetary track but 90 degrees out, far enough to ensure its gravity didn't really mess with the Earth's rotation or orbit. What a wonderful sight seeing it go across our sky during the day. A probe to its surface would take only days or weeks to get to. Sounds a bit easier to play with.