I like K:BB ratio as a predictor and also as a general evaluator of performance. Like hitters who don't walk a lot, pitchers with bad K:BB ratios can rarely sustain their success. K/9ip is about as useful, but it's important to note some successful pitchers, like sinker ballers, may not strike out a lot of batters. But successful sinker ball types usually have good K:BB ratios; at leat 2:1 or higher. ERA and *ERA+ are still useful and handy, but you do have to be aware of factors beyond the pitcher's control that can affect ERA. Slugging pct against a pitcher has value too. It's one thing if the pitcher is giving up a bunch of ping singles, but quite another if every hit is bouncing off or sailing over the wall.