Killer Apps for me have been Beat Saber, Vader, and Superhot. They all additionally have high replayability through endless / dojo modes.
Even more noteworthy is ALVR is available and fairly functional, so with a high end gaming PC and a good 5ghz AC capable router you can stream PC VR games wirelessly to the headset. I've gotten Space Pirate Trainer, Rez Infinite, and Skyrim all working to a good degree. Gonna have to mod Skyrim a bit more though, I want better looking textures for sure.
There's also a good sideloading app available called SideQuest, where you can sideload any Android APK. Through that I got Quake VR working on my Rift, and you can mod Beat Saber to get custom music working already. The livestreaming to Facebook is really cool too, though I wish there was a Twitch app available.