From the group above, it really depends on a lot of factors. What do you wish to achieve with your VR application, what is your budget, who is your target audience, are you going for quantity or quality, etc.
Although, from recent experience if your VR app is going to be used by a large number of people and you are going for quantity, then a mobile based VR is the best option for you!
On the other hand, if for example you know that you are going to have a dedicated place to show off your VR app, for example your company is exhibiting at a fair, or you have a VR app to show your clients at the office. In these cases, it is better to go for quality over quantity, and invest in a high end PC, and a PC based VR HMD.
And finally, if you would like to be versatile, take up a standalone version of a VR HMD device. The standalone version can be purchased for a price of a high end mobile phone, but offers a better quality VR experience than the classic mobile VR does.
So if you’re not 100% sure how you wish to use your VR app, start with a standalone version of a HMD device, and build on from there!