Pluto. Just to get a chance to say goodbye :-(

I'm not finished with Earth yet!

I would choose Saturn because of its mesmerising rings. 

Neptune's environment is exactly a hell, dark, stormy and roaring methane sea. However, in the hell sea, you can find floating diamond and diamond rain.

If people can travel to Neptune in far future, Neptune might be the most preferable planet to go for rich diamond.;)