I'd imagine REC works best on the first viewing, what with the late merging of horror subgenres and...erm...interesting night-vision spectacle. I liked the mystery aspect of it best, I think, and enjoyed the late reveal. REC 2 is also very good; starts off feeling like a cheap cash-in, but gradually subverts that and becomes an interesting film by itself. The third one is just a generic zombie movie, and a massive disappointment.
There's also the American version, Quarantine, which is virtually a scene-by-scene remake. I'd recommend The Poughkeepsie Tapes, which is by the same director, and is a surprisingly nasty bit of found footage. I also quite enjoyed Devil as an entertaining slice of silliness. The buzz about As Above, So Below put me off, but I'll be curious to see that when it comes out on DVD.
As we're talking Blair Witch, it's worth mentioning the film that it was accused of ripping off at the time, Cannibal Holocaust
The good thing about a Cloverfield sequel is that they can do it at any time and have it follow directly on from the original, if they want to. There were no characters that would need to return. So I wouldn't give up hope; wouldn't surprise me if they come up with one when Matt Reeves and JJ Abrams get a hole in their schedules (probably when everyone's given up hope).
Edit: forgot Quarantine 2, which I remember being an entertaining little straight-to-video thing, so long as you go in with low expectations.