Is Mercury the most similar to ours in the solar system?
No. Mercury has no atmosphere and it has an old surface covered in impact craters, so it is very unlike Earth. One similarity is that Mercury and Earth both have internally generated magnetic fields. Venus and Earth are very similar in size. There is emerging evidence for active volcanism on Venus, however, its atmosphere is up to 100 times denser than Earth's and is mostly carbon dioxide with sulfuric acid clouds. The surface of Saturn's moon Titan physically resembles Earth's, with mountains, rivers, lakes, and seas. The difference is that Titan's mountains are made from water ice, which is as strong as rock under its surface temperature (-180°C), and the rivers and seas are full of hydrocarbons.