Down on my sidebar, you can see the current standings of the MLB Heavyweight Championship. This is where we assume that if you beat the champion, you become the new champion, just like in boxing.
Since the Red Sox won the World Series last year, the Heavyweight Championship has resided in the American League all season. Of the AL teams, only the White Sox, Mariners, and Tigers have not had a title bout yet. The Blue Jays have held the title the most, winning eleven title bouts out of eighteen.
As we move into the first weekend of interleague play, the Minnesota Twins are the current champs. The Milwaukee Brewers will be the first NL team this year with a shot at the title. Also, if the Brewers win on Sunday, they can keep crown over in the National League for a few weeks until the next round of interleague play. I wouldn't bet on it, though; they'll be facing Johan Santana. Good luck.