PokéPark Wii gives pride of place to legendary and Mythical Pokémon. Most are won by completing all the bonuses of an attraction; they then become the best Pokémon for that attraction. Here is the list, based on Poképédia.

PokémonHow to get it
MewComplete its challenges in Skygarden: the obstacle course, the triple battle and the race
JirachiAll the bonuses in Venusaur’s Vine Swing
ManaphyAll the bonuses in Gyarados’ Aqua Dash
LatiasAll the bonuses in Pelipper’s Circle Circuit
SuicuneAll the bonuses in Empoleon’s Snow Slide
MetagrossAll the bonuses in Bastiodon’s Panel Crush
HeatranAll the bonuses in Rhyperior’s Bumper Burn
GroudonAll the bonuses in Blaziken’s Boulder Bash
CelebiAll the bonuses in Tangrowth’s Swing-Along
DarkraiAll the bonuses in Dusknoir’s Speed Slam
RotomSucceed at Spooky Shoot-’em-Up with all of the attraction’s Pokémon
ShayminAll the bonuses in Absol’s Hurdle Bounce
LatiosAll the bonuses in Salamence’s Air Ace
DeoxysAll the bonuses in Rayquaza’s Balloon Panic
RayquazaComplete its attraction, Balloon Panic

The details of the attractions are on our attractions page.