This is the best I could find on a quick google, which comes down to "Mars" - although at the end of the day, it's the kind of imprecise question that my DS has no time for at all, and thank goodness he hasn't yet met a teacher who couldn't cope...
http://www.universetoday.com/guide-to-space/earth/what-is-the-most-earth-like-planet/

(Incidentally, my DS-then-5 apparently told his teacher in all seriousness that the Vikings had sent up a satellite. I only heard about this much later!)

Email: my username, followed by 2, at google's mail