If we consider price as a starting criteria, some Google Cardboard clone will do for short sessions (not very comfy) and will only set you back about 10-20$ (but get one with straps!).
For a little more, you can go plastic (using good foam protection) with a chinese unit. There are quite a few of those available (with new variations popping up every week), so it's hard to keep track of them all. ColorCross is popular, because it was one of the first ones. There's the xiaozhai, which is more comfortable (ColorCross uses rubber instead of foam).
As I say, plenty of unbranded variations, so probably newer models work better. Might be good to search on Amazon for the reviews. The main issue with the chinese models I've tried is that immersion level tends to be lower (due to lower FOV), but on the other hand you might get better image quality (less screen door effect, for example).
Then if you want to go premium, you can check the Durovis Dive, the Homido or the VR One. I've used the last two, and they are on a different level, quality wise. Very good results (more immersive). The Homido is limited on max screen size (it will fit bigger phones, but not use the whole screen) while the VR One is, at least for now, only compatible with Galaxy S5 (some people have adapted the provided tray to fit their phones, but that's a DIY that might not always work).
There are actually a few more options on this price range, but I can't remember... Vrizzmo? Cmoar... truth is it's hard to keep up, since everybody is making headmounts these days
I think you can get a quite enjoyable experience with just one of the low cost options (a chinese mount with good reviews maybe). But for better immersion levels, I guess I'd go with a Homido.
Another option (hybrid) is getting a very cheap Cardboard just to get some initial impressions and decide if the limitations are fine or you want something better, and how much better, just a chinese model or a premium mount.
So... I guess after this long post I've just left you more confused than before
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