One of the bucket list items I was able to check off the list a few years ago was to dine at the famous Rao's Italian restaurant in NYC. For those unfamiliar, it's generally ranked at or near the top of the most difficult reservations to get in America. I won't go into the details here but you can Google it for more information. Bottom line, you're not getting in! Read all about it here.
I would consider some of the Blanton's bottles to be significantly less obtainable than a Rao's reservation.
So what's my list? Well it's easiest to start out with what's not rare. Despite the difficulty you might have finding the original 93 proof variety on your local liquor store shelves, it's not rare at all. It can be found easily, often at inflated prices, on the secondary market. The fact it is actually still sold in retail establishments alone makes it common. Even the elusive 50ml or 375ml varieties are out there for the taking.
Furthering this list of commons are the international verities still in production. All of these can be easily obtained online, especially if you don't mind shelling out a few bucks.
Straight From the Barrel
After that, it gets progressively more difficult to find bottles. Silver went out of production in 2009, but you can still buy it today at a few online stores for $1000+ if you really want it. Moving beyond Silver you get into bottles that had limited runs or were produced specifically for the LMDW and M&P festivals.
For the festival labels the more recent bottles will be generally obtainable, while older ones don't surface very often. The older you get, the more uncommon and therefore harder to obtain.
Some bottles are just impossible to find. Even within the network of friends I have in this community I'd say only a handful exist or are even known to exist. I know a few people who have 98% of the bottles listed on this site, but have yet to find someone with them all. They have hunted for years, and likely cost isn't a huge factor should they run across one of the true unicorns they are missing.
Here's my list of rare bottles. It's subjective, but is based on my own research, and conversations with those who have been at this longer than I have:
2012 La Maison du Whisky "Vintage Single Barrel" - I still have not met someone who actually has one and the only picture I have is very low quality.
Sterling / Starling Silver - A Japanese market thank-you gift that was given to liquor store owners. Possibly 100 or less made and only a few known to exist.
Tezuka 100th Anniversary - A thank you gift to employees of Tezuka Corporation for their 100th anniversary. Possibly only given out to higher ranking employees only so quantities were very small.
My Only Blanton's - Both the Individual and Co-owner releases were very low production. One was recently found in Japan that popped up in a liquor store but these bottles remain very elusive and are highly coveted.
Special mention: Green and Black mini's, Older LMDW varieties, Memory of Yujiro, Frankfort 200th, Süddeutsche Zeitung Magazin, and the Special Reserve Brown/Dark Red and no color labels.
My advice is to enjoy the hunt but don't fall in love with the idea of collecting them all. Certainly don't even think about it unless your pockets are really, really deep. Although I'm not a pricing expert, I've easily seen some of the more rare bottles going for $2k+ and the ultra rare bottles listed above are much higher!