Here is the full list of all 16 Cars: Mater-National Championship achievements worth 1,000 gamerscore.
Won every event, collected every Bolt Banner and earned every sticker
Finished in the top three of the Monster Truck Waypoint final
Won all four race in McQueen's new Racing Stadium
Unlocked all 8 of McQueen's new wheel sets, available at Luigi's Casa Della Tires
Unlocked all 8 of McQueen's new paintjobs, available at Ramone's House of Body Art
Completed all six levels of Tractor Tipping
Completed all nine levels of Doc and the Law's Race 'n' Chase
Completed all nine levels of Fillmore's Fuel Frenzy
Won all nine road races, three in each of the three main areas
Completed Ornament Valley and unlocked Tailfin Pass
Beat all seven levels of Ghosting Mater
Found the six parts necessary to construct Monster Mater
Found the parts necessary to construct Monster McQueen
Won the Rustbucket Race Series
Completed all seven levels of Ramone's Rhythmic Rumble
Completed Radiator Springs and unlocked Ornament Valley, and got the sticker to prove it