WASHINGTON — The day before this new season began, Yoenis Cespedes, the Mets’ slugging outfielder, surveyed his team, its health and the new manager, Mickey Callaway.

He declared this team better than the one in 2015 that reached the World Series. And after the Mets toppled their divisional rival, the Washington Nationals, 6-5, late Sunday night to cap a three-game sweep, Cespedes repeated his claim. This time he had a little more proof.

“In my six years in the major leagues, this is the best team I’ve ever been on,” said Cespedes, who drove in the winning run in the 12th inning. “We’ve been showing it, and we will keep proving it.”

The Mets, whose implosion in 2017 is still bitterly remembered, are now one of the winningest teams in the major leagues. Entering Monday, the Mets, the Houston Astros, and the Boston Red Sox were the only teams that had won at least 80 percent of their games this year. And with a 4-2 win Monday night against the Miami Marlins, the Mets improved to 8-1 for the first time since 2006 and matched the franchise record for best start to a season.