Choosing which partners to upgrade first in Paper: Mario The Thousand-Year Door will depend on what you continue to do in the game. For example, if you are looking for completion, higher damage characters by the game’s last chapter should be prioritized. Additionally, characters that can give Paper Mario extra effects should be used in every battle. These skills, such as Tattle and Rally Wink, used by Goombella, will probably be some of the first Ultra skills you unlock.

https://screenrant.com/paper-mario-thousand-year-door-best-party-member-upgrade-order/