Which planets is least like earth in size and composition?
If I had to pick one planet, I would say Jupiter because of its exceptionally large size and mass. But all the outer planets Jupiter, Saturn, Uranus and Neptune are highly unlike Earth.
There are really two kinds of planets in our Solar System. The mostly rocky ones include Earth and three others (Mercury, Venus, Mars) that are more or less similar in interbal composition. The large, gaseous, hydrogen-rich ones -- Jupiter, Saturn, Uranus and Neptune -- are all so unlike the rocky planets that calling one of them "most unlike Earth" is really splitting hairs.