It very much depends on what you want to do. For low-cost VR, I’d use the GearVR. The main issue with GearVR is that it does not have positional tracking - but at least it has pretty good rotational tracking (unlike even lower-cost solutions like Cardboard VR which I personally really dislike).
Oculus DK2 or OSVR have positional tracking and are pretty okay to work with. With Oculus DK2 you have the benefit of being able to use Unity’s native integration (which also works for GearVR, of course - but you have to do builds and put them on a device to test, while with DK2, you just hit play, put the headset on and enjoy 
). The OSVR HDK should also be pretty easy to get and can be used with the SteamVR Unity plugin (see below).
All of these, however, lack motion tracked controllers. If you can still get your hands on a Razer Hydra, you at least have some sort of motion tracked controller but the tracking is meh, so I wouldn’t really recommend it.
IMHO, the “gold standard” for VR is currently the HTC Vive and they’re about to give 7000 units of their DK2 to developers. Their DK1 is developer heaven - it’s just a completely different kind of “VR” because you have reliable low-latency 360° room-scale VR tracking and you also have nice motion tracked controllers.
With the HTC Vive, you could still develop for standing or seated experiences - but you’re not limited to those. They have a plugin for Unity which IMHO is the best VR integration into Unity (I prefer it over the native integration because I really don’t like the “just tick a checkbox and your game supports VR approach which imposes quite a few limitations IMHO”). The SteamVR plugin is available on the Unity Asset Store.
The only challenge here is actually getting one. They don’t cost you a dime - but they only select developers that they believe will deliver something awesome. Current release date for the consumer version (which you could also use for developing, of course), is April 2016, with pre-orders announced for February 2016.
Finally, there’s PlayStation VR which would be my second choice after HTC Vive. They do support forward-facing seated or standing and they do support motion tracked controllers (their headset has 360° tracking but you will occlude the controllers when you turn around, hence “forward-facing”).
I also believe PlayStation VR will probably be the biggest market for VR because a lot of people already have a PS4 (unlike people with PCs that are capable of properly running VR). Unity also has built-in native support for PlayStation VR. The tricky thing is that you need to get PS4 developer hardware to be able to use that (and then you need to be selected for their development kit). Sony’s pretty strict with NDAs but I’d say you need a company and a bit of funding if you want to develop for PlayStation VR … but I think it’s certainly worth it.
