I discovered this particular chart at www.baseball-reference.com which indicates what current and former teams have the best won-lost percentage since the franchises began.
OK, for current teams, you likely guessed the New York Yankees, formerly the New York Highlanders and Baltimore Orioles are #1 by about 30 percentage points. But who's #2?
Interestingly, the Yankees are NOT the #1 team in terms of Hall of Fame members. They have 42, but the Atlanta Braves (a .500 team since 1876) have 45; the Los Angeles Angeles Dodgers (#3 in winning percentage) have 46.
The #1 team in term of Hall of Fame representatives are the San Francisco Giants with 55; the Giants have the second-highest winning percentage.
The current teams with the lowest winning percentages are Tampa Bay (by a wide margin), followed by San Diego, Seattle, Texas, Philadelphia, Baltimore, Washington, and Milwaukee.