When you gain a level in Paper Mario: The Thousand-Year Door, you'll be given a choice to raise one of three stats for Mario. There are Health Points (HP), Flower Points (FP), and Badge Points (BP). Unfortunately, it's impossible to have all three maxed without the help of Badges. There isn't a "best option" for leveling up, and the stat you choose to increase should be based on your preferred gameplay/strategy. However, one stat may take priority over the other when you're just starting your playthrough. As you advance through Thousand-Year Door's campaign, you'll have a better idea of what to select upon leveling.