First of all, Pluto isn't a planet. The Moon is more of a planet than Pluto.

The question is obviously subjective, but for 6/8 planets it comes down to "How scary is sudden death?" Because that's what awaits a traveller with our current level of technology. Mercury is practically half on the sun, half deep space. Venus is a basically hell, fire and brimstone doesn't begin to cover it. The gas giants are collections of constant storms. Mars is literally the only other planet we can go to and not die in an hour, but we still don't have ways of making it habitable.

Earth has a lot of downsides, but this is the only planet where Reagan, Thatcher and Kissinger died.