This Shout PINK PANTHER box has to be at the top of the list. New copies sell for $400, used above the $200-$250 mark...hard to think of another "retail" release as worthwhile at this stage! (Except Shout's HALLOWEEN box-set, which is even more in-demand)
Incredibly it remains the only release of all the Peter Sellers Panther films on Blu-Ray anywhere in the world -- and may end up being the only one considering the MGM sale to Amazon. (Unless another label is going to get involved in the very near future).
There was a solo release of the first film from MGM/Fox, but in the U.S., the Sellers sequels never saw a release outside this one (just as an aside, CURSE, SON and the Arkin INSPECTOR CLOUSEAU are available still from Kino Lorber).
That lavish new Arrow release might drive the price for that down, but it's still indicative of the value of physical media in the era of streaming. There are a number of Kino titles I ended up not caring for when blind-buying them, yet I'm keeping them in a drawer somewhere while they quietly go OOP and suddenly what I spent ten bucks on will be going for eighty on eBay. 
). Once a movie or CD is on my shelf, aside from my apartment complex buring down or disc rot setting in (which has only happened on a very small number of CDs/Blus over the last 20+ years), those are MINE, baby, and they'll never be taken out of my hands and mutilated to remove "problematic" content. 
Basically, if it's a movie you might want to see at least once more in your lifetime, it's worth buying an actual copy of. And even if it's not a good movie...buy it anyways for future selling/trading purposes, if there's even a chance of it going OOP. Hell, JoJo Rabbit, an Oscar-winning film that hit Blu and UHD all of a year-and-a-half ago, is already getting scarce. 
