It really depends on what you want to do, what the size of your target audience is and what budget you have.
The VR applications for Google Cardboard and Homido, can be developed for all mobile phone platforms and optimized in terms of content. The great advantage is that apps can be used by large number of people, and HMDs are at lower end in terms of cost. But if you need more control over content, and better quality of graphic you might opt for Samsung Gear VR, in which case you can invest in Samsung VR Gear and matching Samsung phone (on Android platform).
Oculus Rift and HTC Vive require dedicated PC platform, hence they come at a higher price tag. Still, they can support high quality graphics and offer greater means of control than HMDs intended for mobile platforms. VR applications can be made for PC platform which are dedicated to specific company use case and targeted at limited users, at present.