Depends what you're looking for. If you're looking at how well a pitcher or batter performed, the best stats are wOBA (hitters) and SIERA (pitchers). If you're trying to predict outcome, SIERA is also good. But if you're trying to predict performance* you can't possibly use a single stat. You need to look at trends, like velocity, swinging strike rate, etc. For hitters, a change in FB% and an increase in HR could support a hypothesis that a player has changed his swing to make it more of a "home run swing". So basically, wOBA and SIERA are the best way to measure past performance, but to predict the future there is no one stat you can use. You have to look at the big picture.
*performance is different from outcome in that outcome is what actually happens while performance is what that one player did isolated from any external factors, e.g. wins is an outcome stat and ERA is its conjugate performance stat; ERA is an outcome stat and SIERA is its conjugate performance stat. Basically the outcome stat is explained by performance stat + fudge factors like team defense or run support.
Rocinante2: you know
Rocinante2: its easy to dismiss the orioles as a bad team
ofanrex: go on
Rocinante2: i'm done