Which Other Planet (or Moon) In Our Solar System Is Most Like Earth?

It's a toss up between Venus and Mars.  Venus ought to be the one.  It's about the same size, and has a similar amount of atmosphere.  Although it's closer to the sun, if Venus had the same atmosphere as Earth, it wouldn't be that much hotter than Earth is.

The problem is that Venus has a horrible atmosphere: lots of carbon dioxide, methane, and other noxious gases (for life forms on Earth).  The high concentration of methane and other 'green-house' gases means that Venus retains the solar energy that comes into it too well, so has a typical surface temperature of 800 degrees C.  The air is unimaginably heavy and hot.  If you stood on the surface of Venus you would be crushed, asphyxiated and fried simultaneously.

Mars is a next best candidate.  It's much smaller than Earth, so has less gravity.  Accordingly, it also has less atmosphere... and no surface water.    In spite of Stephen J. Hawking's pronouncement that mankind will have to immigrate to the starts to save itself, it really isn't clear where we could go.

This has always been a tough choice between Venus and Mars but in my opinion, Mars is a better choice and this is because Mars is the planet which is much more similar to earth than fron Venus. Mars has been explored and it has been found out that Mars has Ice caps at its Poles also it has been observed in the planet that decades back the planet had water all over but due to some circumstances it had all turned into the ice caps at the pole.

It is also said to be a better choice because it has an atmosphere all around which has Oxygen (but in a very less Quantity ) more of carbon di oxide is there but Venus does not have any atmosphere. Also traces of methane have been found that tell about the micropbes that may have existed before as life.

Mars also has a mass very similar to earth and the radius of it is almost similar. Mars is also at a distance from sun similar to that of Earth thus it is given as a Hypothesis that Mars may had ever sustained life forms and therefore it is considered to be the most similar planet to Earth.