I won't list the expensive ones but some of the cheaper ones would be:

Forbidden Planet, Criterion LD Set
Dune TV Version, asian LD Box
The Thing, asian LD
Nightmare on Elm Street 1, Elite Special Collector's Edition LD Set
NOES 5 uncut LD and some of the series on jap LDs...
The Frighteners, Signature Collection LD Set
Death Machine Japanese 131min Director's cut, 2 LD Set
SP/FX: The Empire Strikes back, Japanese LD
First Strike, asian LD
Flesh and Blood, US 2 LD Set

JM (Johnny Mnemonic) original uncut asian DVD
Lifepod asian DVD

LEXX, workprint of the pilot on tape

I have quite a few Criterion thingys on DVD and LD and Pioneer Special Edition DVDs like the Bride of Reanimator or TCM... but what I used to watch over and over again are the Star Trek TOS LDs, I just love them

I also like the old star trek definitive collection but I wouldn't consider it rare as it's sold on ebay on a wekkly basis