For me it was the first time I played Richie's Plank. Being able to fly above and around the city made me feel like a real super hero. Back then, it was not as pretty as it is now nor did it have as good physics and object interaction, but it was still very magical. I felt like a kid again.
Runner up is the first time I played Thirst VR. I got it mainly because it was on sale at the time for 98 cents, so I figured why not. It had positve reviews so I didn't even look at any pictures or anything, just figured it was too cheap to pass up. One play through and it suddenly dawned on me, I'll be able to say one of my all time favorite VR games cost me less than a dollar. It looks just like Madmax, in VR. I still play it a lot, and because you stand still and the game mechanics are simple, it is a great intro to VR for people who come over as it doesn't induce motion sickness at all and I don't have to worry about them moving around too much. But that first moment, when screen went blank and the game started and I suddenly realized, this is Mad Max VR? Priceless.