Answer: the most representative one is Las muerteadas y comparsas
Explanation: The question says:
"What is one of the outstanding parties in the city of Oaxaca?"
There are several ones, for example the carnivals of Silacayoapan and San Martín Tilcajete are the ones more famous.
And maybe the most "representative" party is the "Las muerteadas y comparsas" celebrated the 1 and 2 of November, where the people is celebrating the "day of the dead" with dances and music.