By Fangraphs reckoning, he's the most "clutch" player so far this year and it's really not close.
They define clutch by comparing how a player normally performs vs. how he performs in high leverage situations. It is a past looking stat and is not predictive. The way it works, if you are a guy who normally is a .240 hitter with weak power but hit .300 with above-average power in high leverage situations, you will likely do better on it than a guy who normally is a .320 hitter with very good power who hits .330 with very good power in clutch situations. Thus, Harper is more "clutch" than Trout, but Trout has probably performed better in clutch situations.