After the Earth, Mars is the most habitable planet in our solar system due to several reasons:
The only other two celestial bodies in orbits near the Earth are our Moon and Venus. There are far fewer vital resources on the Moon, and a Moon day takes a month. It also does not have an atmosphere to form a barrier against radiation. Venus is a veritable purgatory. The average temperature is over 400 degrees, the barometric pressure is that of 900 meters underwater on Earth, and the cherry on top comes in the form of occasional bouts of acid rain. It also has nights that last for 120 days. Humans cannot live on Mars without the help of technology, but compared to Venus it's paradise!