This is a great question. A lot of the Pokemon I've used in my run of Reborn have been unexpected finds, yet also very helpful, which is to be expected when you decide to limit yourself to only using Pokemon you've never used before or haven't used in a while. If I must pick one, I cast my vote for Swalot. It was a major help against Florinia, helped me beat Corey in 1 try when I was struggling to not lose my sanity against the Gym Leaders early on, and its defenses make it an amazing tank while it slowly takes down the opposition and regains health with Black Sludge.