New York Mets: 1969 World Series "Miracle"
Chicago Cubs: Wrigley Field
St. Louis Cardinals: Stan Musial
San Francisco Giants: Willie Mays
Los Angeles Dodgers: Sandy Koufax (maybe Jackie Robinson if you count their Brooklyn era)
Detroit Tigers: Al Kaline, Denny McLain & the 1968 World Champions
Pittsburgh Pirates: Roberto Clemente (Maybe Bill Mazeroski or "We Are Family")
Cincinnati Reds: 1970s "Big Red Machine"
Oakland Athletics: 1970s "Mustache Gang"
Milwaukee Brewers: 1980s "Harvey's Wallbangers"
Toronto Blue Jays: Joe Carter
Baltimore Orioles: Cal Ripken and his streak
Seattle Mariners: 1995 American League Division Series
Washington Nationals: 2019 World Series
But not every team has a good "first thing you think of":
Chicago White Sox: Sorry, after 100 years, still the Black Sox Scandal
Cleveland Indians: Sorry, still choking
Los Angeles Angels: Sorry, even after 2002, it's still choking
Philadelphia Phillies: Sorry, even after 1980 and 2008, it's still choking
Miami Marlins: 1998-98 First-to-worst
Atlanta Braves: That stupid Tomahawk Chop
San Diego Padres: Awful uniforms
Minnesota Twins: Home-happy Metrodome (good riddance)
Colorado Rockies: Homer-happy Coors Field
Texas Rangers: Taking nearly half a century to block Texas' heat with a dome
Tampa Bay Rays: Worst baseball stadium of the modern era
And it could be worse:
Houston Astros: Cheating with electronic sign stealing
Boston Red Sox: Cheating with steroids, and maybe also with electronic sign stealing
Kansas City Royals: Cheating with pine tar
Arizona Diamondbacks: Ruining NY's post-9/11 comeback with a tainted World Series