Not one who really follows rare films to own. But the Avatar steelbook edition blu ray I got a few months back went out of print the next day... so the day I pre ordered it I got it for around £19, the most I've seen it going for now is £160...
Movies are something that can't keep their worth because it's so easy for companies to just put out another copy. I remember when the Mystery Science Theater 3000: The Movie DVD was worth top dollars. Then the remastered DVD came out. Sure enough, the value of that DVD went straight down.
I have some Steelbooks that may be considered "rare":
Appleseed and Appleseed: Ex Machina
Grindhouse: Death Proof and Grindhouse: Planet Terror
I Am Legend
Iron Man 2 (Blu-ray metal case)
Star Wars: The Clone Wars (movie)
Watchmen: Tales of the Black Freighter
I would have mentioned Spawn: The Animated Series and The Texas Chain Saw Massacre, but I think those Steelbooks are still being made. I've never seen them on the shelf in a standard jewel case.
And speaking of Watchmen, I think The Complete Story is out of print now, so my Blu-ray of it might be considered "rare."
There are many editions of the Evil Dead trilogy out there. I have the Book of the Dead editions of the first two and the Boomstick Edition of Army of Darkness. I think all three of those are long out of print.
I have the National Treasure Presidential Edition (5-Disc set). I think it's pretty hard to find these days, but I don't know if it's particularly sought-after. I got it for around £20-£30 back in 2008 but I've seen it go for around £75 since then.
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It's made out to look exactly like the President's Book in the second film so it's really neat.
One that I never really understood the desire to have was the Matrix "ultimate edition" with the model of the Nebuchadnezzar....It looks like a giant woodlouse...Horrible design and not exactly the focal point of the film so why bother?