100 years ago nobody would have thought the USA would have had people on the Moon or Russians would have men living orbiting our Planet.
Let's assume in 50 years time we have developed great propulsion from developing technology such as fusion, ion-drives, nano-tech, anti-matter or fisson. Rather than waiting months or years to reach other world it takes just a matter of hours or weeks.
Now there is a new space race launched by all the space agency's to build settlements or bases on every world in our solar system. Which of the worlds would you consider to have the highest levels of hosility and be very hard to inhabit. For the puropose of this discussion I'd like to stay away from Asteroids, Exo-planets, Gas-Giants and Comets.
Here is my list of Ten
1 Io : Unlike that Outland movie Io would be almost impossible to colonise. It has huge variations in temperature, it doesn't have a stable surface and is the most volcanically active body in our solarsystem, it has a thin toxic atmosphere and its ionizing radiation would be instantly lethal
2 Mercury : Forget Ben Bova's mercury or Arthur's Rendezvous with Rama, this minor planet is deadly. Flying near the Sun would always be high-risk, Mercury has huge changes in temperature, it has little or no atmosphere and radiation dosage would often be deadly during flares.
3 Venus : This planet has 2 major problems, it is hotter than Mercury and has a highly corrosive atmosphere. However the Russians proved craft could be built to survive this hellish climate, upper-atmosphere is less hostile and planet's surface has rescources.
4 Triton ( Neptune ) or Titania ( Uranus ) these bodies are very cold, they are some of the coldest places in the solar system and lie at great distances from Earth so rescue missions would be almost impossible but they do have solid surfaces and a lot of water-ice
5 Pluto : Cold, Dark, and even with new propulsion it would still be very far away from the Earth.
6 Saturn's Moons : Mimas or Rhea
7 Ganymede or Callisto
8 The Earth's Moon. Russia sent Luna probes to the Moon, the USA sent Rangers and Surveyor's and landed Armstrong and Aldrin with the Apollo mission, it is now the target of ESA's Smart-1 and may soon be explored by robotic craft from China and India. The Moon however is a lifeless rock with few resources, lunar dust could be deadly. Its close proximity however will always be an advantage.
9 Mars : It has a 24 hour day and looked habitable to early astronomers who thought they saw channels and canals, Orsen Wells caused American public panic with his War of the Worlds broadcast. Mars may have once had flowing water but today it appears to be a dead, dusty and cratered world. Martian dust could be toxic and building a long-term colony could be difficult due to radiation exposure.
10 Titan : Very cold and it also has low gravity but Titan seems quiet planet-like and might also be full of rescources.