Let me preface this section by admitting that I have mixed feelings about "VR" delievered on Google Cardboard at this time.

Don't get me wrong, I have a lot of respect for what Cardboard has done for consumer VR.  But I have found the experiences on Google Cardboard to be fragmented and imperfect, at best.  So bad in fact, especially now, that I think Cardboard experiences may actually be hurting the reputation of mobile VR rather than helping it.  

It's relatively early days for Google Daydream, but if things go the way Google is hoping it will go, the VR platform will likely offer a much better experience than Cardboard currently offers. That said here are some apps that I believe are great must have titles for Google Cardboard.

TITANS OF SPACE

This is one of the first VR experiences to make it to Google Cardboard and mobile VR.  Considered a "classic" title at this point, but so polished and informative that I'm still recommending it to those interested in learning more of our solar system.  It's a linear experience that is built in 3D to really help you get a sense of the sheer scale of some of these celestial wonders.  Entertaining, polished, and informative.

YOUTUBE (CARDBOARD)

YouTube is on most mobile devices.  When you play 360 videos, you can toggle it to "cardboard" mode by pressing the cardboard icon in the user player interface.  YouTube is full of great 360 video experiences to enjoy on your cardboard device.

PERFECT BEACH

This is a simple app that could offer more but still quite enjoyable to use.  You can be male or female and basically you're just sitting on a beach enjoying a beautiful day or a gorgeous sunset.  There's nothing much to it beyond that but the reason I like this app is because of the sights and sounds, it delivers an escape mechanism for those looking for it.  It's a great app to have to showcase the immersive qualities of VR.

VR MEDIA PLAYER

This is one of my favorite apps in general.  If you have any photospheres or 360 videos stored on your computer, this is the media player you're definitely going to want to have on your mobile device.  It's simple, extremely elegant, and lightweight.  It's not just a VR app, you can use it with or without Google Cardboard.  Supports a wide variety of 360 videos in different formats... Swipe or gyro sensors or both. pinch to zoom in/out is great, very zippy, excellent app.  A definite must have.

ANDROIDS DREAM

Without saying it (clearly avoiding copyright litigation), this is a sci-fi VR experience has you sitting in a flying car touring the futuristic city.  Recognize it?  Yep it's a nod to the futuristic city in Blade Runner.