Mars is the planet that has received the most attention, with many probes being sent over the years. This is probably because many people see this as the only candidate for being colonised and possibly terraformed in the future. There is also much interest in seeing if there was once life on Mars.
Over the years, whilst we have had a lot of successful missions, there have also been many failures. However several rovers have now studied parts of the surface in some detail and all the planet has been imaged to high resolution.
Mars has the largest volcano in the solar system, Olympus Mons and the largest valley, Valles Marineris