The problem with the rarity lists that include TI cartridges is that they often rate rarity by feel--not by actual rarity. Very few of the TI produced cartridges are rare, with the three rarest production cartridges being Computer Math Games III, IV, and I (in increasing order of rarity). III shows up on eBay once or twice a year, IV shows up once every year or two, and I haven't ever seen I turn up (though I have seen a couple of them in friends' collections. There were a lot of near-production prototypes that truly deserve the rare designation though. Disk Manager 3 fits into that category.
The Scott Foresman labeled cartridges include quite a few rare titles: any of the School Management Modules and the three combo game modules (Module A, B, and C). A number of them also had variant labels by a company called Tronics--apparently, not too many were sold via that route, so the cartridges were relabeled with the SF logo labels (white with red text instead of white with blue text). Not all red text modules are rebadged Tronics copies, but most are.
None of the Atarisoft modules are particularly difficult to find, same goes with the Parker Brothers, Navarone, and most of the Funware titles. Video Vegas and Schnoz-Ola are the two hardest of the Funware titles to obtain, and both of them deserve the rare rating.
Most of the Asgard modules are difficult (though a lot of them have been showing up on eBay lately--but the majority of the ones I've seen lately are newly produced pirated copies, not originals). All Asgard titles combined would equal a sum of about 500 cartridges total. I used to hang with the owner, and he only bought one lot of 500 empty cartridge cases from TI to fill with his titles. . .
Several of the Navarone titles were pirated by a dealer in Holland several years back as well--those are easy to recognize as he put them into Funware cases.
DataBioTics titles are almost always uncommon to rare with the exception of a few of the game cartridges and Typwriter, though it appears that these may also be in current production by the same source pirating Asgard cartridges. I don't have sufficient data to be sure yet. . .but I'm working on it.
One note: TI cartridges produced in the European Market for distribution there are much more difficult to find than the US titles--and some of the European titles are unique to Europe (these unique titles would definitely fit into the rare category)