In terms of direct observation:
* Uranus and Neptune are the two planets we know the least about in terms of direct observation. They're incredibly far away, making it difficult to study them with telescopes. We've only had one spacecraft visit each: Voyager 2.
In terms of understanding:
* Venus is a planet we still have many mysteries about. Its incredibly dense atmosphere and extreme surface temperature make it very difficult to study.
* The gas giants (Jupiter, Saturn, Uranus, and Neptune) are vast and complex, with many unknowns about their internal structures and atmospheric processes.
In terms of potential for life:
* The outer moons of Jupiter and Saturn are fascinating because they might have underground oceans, making them potential candidates for harboring life. However, we still have a lot to learn about these moons.
Ultimately, there's no single "least known" planet. We're constantly learning new things about all the planets in our solar system, and there's always more to discover.
