Vanishing Realms, easily. It's short, like essentially everything so far, but it feels great. The only complaint I have with it is items not always having physics is... jarring. Nothing deal breaking obviously, but being able to toss objects around in games like Job Simulator, The Lab, and Budget Cuts really sells the authenticity of the world. A lot of Vanishing Realms feels static, because it literally is. That said, it's an amazing experience, and it's one of the few Vive titles that actually feels like a proper videogame.
This doesn't reflect the bundled games or free stuff, obviously. I think in that realm, The Lab is the best experience you can have with virtual reality right now. Valve absolutely killed it, everything in that package is flawlessly executed. Even the most basic experiences like Robot Repair are mindblowing.