A planet roughly the size of Earth orbiting within the comfort zone of its star. This would give it a nice field of gravity and temperature, plus the chance for having a good magnetic field. But lets say that the atmosphere, although ok pressure wise, is made mostly of carbon dioxide or some other gas gas mixture unsuitable to Humans.

We have the technology today to break such gases down into their basic components By using such technology and enclosed habitation, the colonists could get a toehold on the planet and begin using microbes and plants to make the atmosphere breathable. Such an arrangement, while not ideal, would still be bearable.