On topic: My rarest DVDs are probably the three Lord of The Rings giftsets, A Better Tomorrow Trilogy limited edition set or my Die Hard Trilogy Five Star Region 1 set.
Edit: Actually my rarest DVD is the out of print, Region 2 original pressing of True Lies. The original release used the uncut master by mistake, this was quickly recalled once the error had been realised, and was replaced with the UK cut version. I managed to pick up the uncut version from eBay for about £6.00.