There's a reason baseball teams play 162 games: It takes that long to figure out who's good and who isn't. Jumping on or off bandwagons based on the first three weeks of the season can lead to little more than busted ankles. There are too many variables that can distort a team's record to draw reliable conclusions.
The most significant of those is the schedule. Take the Red Sox, whose first five series were against four 2011 playoff teams and the strong Blue Jays. Boston started 4--10, but a road trip to Minnesota and Chicago helped them put together a six-game winning streak and briefly get back to .500. At the other end of the spectrum, the three NL division leaders—the Nationals, Dodgers and Cardinals—have played three of the four weakest schedules in the majors. We simply don't know if they're as good as they look, or just taking advantage of a steady diet of cupcakes.
The charts below rate the best and worst teams so far based on their competition. (This early in the season, a team's projected record is still a better indicator of its quality than its actual record, so actual winning percentage was weighted at 25%, and projected winning percentage at 75%, to create a weighted opponent winning percentage, or strength of schedule rating—SOS.) Taking schedule into account, the two best teams are the Rangers and the Rays, who have both played great baseball against above-average competition. The surprise is the Mets, whose 12--9 start came not at the expense of patsies but against the fourth-toughest schedule in the game.
THIS IS AMAZIN' Daniel Murphy and the Mets are off to a fast start despite the fourth-toughest schedule.