Fusion Evolution: Polyform2
Typing: Normal
Stats: 98 / 75 / 90 / 91 / 90 / 65
Ability: Aeroform - Changes forms and typing to Fire if Sun, Water if Rain, Ice if Hail, and Rock if Sand for the rest of the battle. This Pokemon then obtains the respective weather-setting ability for the rest of the battle, which will force-activate.
Reasoning behind nomination: Polyform2 is an interesting weather Pokemon, but it is quite weak, slow and its typing isn't that good either. It needs weather support to enter one of its forms, which means you have two weather setters on your team and I don't think people would like to waste team slots like this. You could use Trick Room to help with the horrendous Speed, but Polyform2 is still very weak and it is unable to break the monster walls we have in FE.