Final answer:
The National Action Party (PAN) won the Mexican presidency in 2000 against the PRI, signaling a shift towards a more competitive democracy.
Step-by-step explanation:
The opposition party in Mexico that won the presidency in 2000, breaking the Institutional Revolutionary Party's (PRI) long-standing hold on power, was the National Action Party (PAN). PAN's candidate, Vicente Fox, was the first to defeat the PRI since it began dominating Mexican politics in the 1920s. Although the PRI was reelected in 2012 with Enrique Peña Nieto as president, the 2000 election victory by PAN marked a significant shift in Mexico's political landscape and a noteworthy moment in the country's move towards a more competitive democratic system.