No planets besides Earth have air that we can breathe, so in that sense, no other planets are inhabitable.
Of the eight planets, only Mars, Earth, Venus, and Mercury have a surface you could stand on.
Only Mars and Earth have surface temperatures low enough that a habitat could be built there with current technology.
But such a habitat could indeed be built on Mars. If, right now, advanced aliens made it easy for us to travel to Mars but gave us no help in surviving there, it would be easy enough to build a pressurized habitat that people could live in, with regular restocking of supplies.
Making such a habitat self-contained would require substantial research and development. The atmosphere is very thin and almost all carbon dioxide. The soil is not hospitable for growing crops. No particular obstacle seems insuperable, but an entirely self-contained habitat, even starting with Earth soil, has not been successfully created on a human-livable scale. (Biosphere 2 was a famous attempt.)