Felix Hernandez has one of the best fastballs in all of baseball. Last year, opponents hit only .183 against it, best in the majors among starters. And no other starter held opponents below the Mendoza line with fastballs.
King Felix's change is equally devastating. Against left-handed batters, right-handed pitchers tend to pitch the change away, attempting to get the batter to chase. This doesn't always hold true when pitching against righties. As a changeup from a RHP tends to move in on right-handed batters, it's sometime safer to start the pitch inside in order to avoid floating a pitch over the meat of the plate. If you have a great fastball, you are more likely to get away with throwing a changeup in as well. Felix is a prime example of this.
Righties produced a .117 wOBA against his change with an expected line of .139/.204/.169 last season (compared to an actual line of .129/.126/.130.).