One of the most exciting VR demos that I have yet to lay my eyes on is the Valve VR demo that is Portal-themed, running on the HTC Vive headset. I've tried nearly everything else, but this is the one that everyone has talked about being the best. Check it out in the embedded video below.
As you can see, Valve has an excellent room-based VR experience, that really shows off a great play through VR. You can explore every corner of the Aperture Science lab, pulling drawers and levels like you were right there in the room. I'm sure that this demo is merely a tease of things to come from Valve, when we eventually see Half-Life 3, Portal 3 and Left 4 Dead 3 in the [hopefully] near future.