Oh man, great question... I like fablun's way of breaking it down, too.
Some recent answers for me would be:
Complete set of the Japanese Kanzenban edition of the Silent Mobius manga. It's not "impossible to find" rare (very few things truly are in Japan, if you're willing to pay the money), but it is out of print. Was the first order I had re-shipped to me through Tenso, so from that point of view as well was satisfying.
Taiho shichauzo (You're Under Arrest) Movie on Blu-ray - the improvement in picture quality is astounding, a great advert for the value of continually improving technology and something I totally wasn't expecting to be as much worth buying.
Funimation's Dragon Box releases. Never in a million years expected to be able to buy these, even with how much Dragonball stuff has seen a release over time. For all of Funi's growing ranks of minus points, this release is an amazing gift to fans of the show that largely made them what they are today.
Will have more of a think about other stuff... and safe to say that if the first set of Durarara!! arrives any time soon, it will be worth a place on my list.