I typically play Space Pirate Trainer first; once I get around level 15-20 things are so hectic I'm moving a ton and often going to one, or both knees. My abs and back can feel it big-time. I start with this game because it's not quite so intense at the start and is a good VR warmup.
More impressive is Holopoint - a bow and arrow game. That is easily the most physically demanding VR activity that I've found so far. I'm usually sweeting pretty solidly when I complete 8-10 games of Holopoint. I'm also noticeably fatigued in my arms, back, legs, hips, all over. And just to be clear most would classify me as extremely fit (regularly skate with/against NHL bound Junior players, the minimum pull-ups I do in my workout are 30 consecutive, body fat <10% etc).
Lastly I find I am no longer interested in 'regular games'... such as Madden, NHL 16, Gran Turismo, etc (on PS4), or even my all time favorite Dark Souls (series). I simply can't go back to not being physically engaged the way VR games are.
VR is going to be absolutely huge in the health/fitness space.