I recently began playing Paper Mario on Nintendo 64, and I have gone through pretty much the entire game. Who would the most powerful sidekick be to use in the final levels? Right now I'm about to face Huff n Puff (?). Also, any easy ways to beat him?
The sidekick that you get for a particular stage tends to be the best for defeating the boss of that stage (and many times the stage itself) at least if you're playing the game normally and the sidekicks are equally upgraded.
Stage 0: Goombario is the only sidekick, so what do you want?
Stage 1: Bombette works perfectly for normal enemies in this dungeon as well as the bullet bill launchers. Kooper's multi-target attack is great for the multi-part boss, and dizzy shell prevents them from going into super-attack mode if I remember correctly.
Stage 2: Parakarry is perfect for this boss with his powerful ranged attack, and ranged attack in general.
Stage 3: Bow protects Mario from the Tubba Blubba's Heart's charge-round attack with her invisibility.
Stage 4: This boss has several parts. I don't remember the first couple parts, but I think Goombario, Kooper, Bombette, or Bow may make quick work of them due to superior attack. Watt is best for the final (main) part of the boss due to the fact that he(?) ignores defense with his attacks and the boss has 2 defense.
Stage 5: If you upgrade Sushie to ultimate form, she is unstoppable for this boss since she can stun all three of the boss enemies at once. Even if you don't upgrade, she is still the only partner who can stun even one of the pieces other than Mario (and he requires equipment to do so).
Stage 6: Lakilester's Spiny Surge is great for taking out the Tuff Puffs. It's a cheap move, it does very little damage to Huff N. Puff himself (thus generating fewer retaliatory Tuff Puffs) and saves you quite a bit of damage. Cloud Nine is also great for protecting Mario in this battle.
After that, I think that the partner that really shines against the bosses is Watt. Both of the final bosses have defense, which Watt ignores. Watt can also increase Mario's attack which helps to mitigate the additional defense since the battles last relatively long.
Fighting normal enemies is generally pretty trivial, but if they have no defense, Goombario (Charge + Multibonk = 10+) and Bow (Fan Smack = 10) can cause a lot of damage to individual enemies. Bombette's Ultra bomb is nice for taking out hanging enemies as is Parakarry's Air Raid (easier, but less damage), and Sushie's tidal wave is also good for this. Lakilester and Bow can also get rid of trash if you don't feel like fighting. The only partner I don't feel ages well at all is Kooper. He does really shine in Stage 1, but you probably won't use him much after that. His Ultra move can instantly kill Dry Bones, though, which is nice.