Hey! I'm trying to keep my giveaway thread on topic, so I'll answer here :P There isn't one nature you would absolutely want for a Phantump because it's versatile. a VGC set would normally have a Brave or Relaxed Trevevant, while in singles, Careful is probably the best for a bulky set, or Adamant for an offensive one. If I had to pick just one, I would pick Brave, but all the options are good!