The conventional wisdom of pennant races is that the games against your chief rival are the most important. Unlike a lot of conventional wisdom, this has the advantage of actually being true. Every win your team collects against your fiercest competition is a guaranteed loss for your nemesis.
So it's not a stretch to say that this weekend's series between the Los Angeles Angels and Oakland Athletics starting Friday night will be absolutely crucial to determining who wins the AL West. And it's not just these three games, either -- this weekend's series is only the opening salvo in the showdown, with the two division favorites facing each other for two more series after this.