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
Hancock
I Am Legend
Iron Man
Iron Man 2 (Blu-ray metal case)
RoboCop
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.