I'm "in the game" and religiously watch what becomes available and what doesn't. CFE223 isn't that hard, it often comes up for sale at least weekly and stays up for far more than a few minutes. I'm not saying, like anything else, it stays up for long. But I do see it often.
Varget is one that is still coming up for sale, but it's one that gets sold out within seconds when it does. When I say "in seconds", I mean literally less than a minute.
Now that I've been watching and actually have someone waiting to buy, I'd say Retumbo is barely being made anymore. I'm thinking H1000 is meant to be its replacement. So for me, that's the "rarest".
I'm sure there are several powders that people are waiting for, but demand just isn't that high so the manufacturers aren't pushing it out. VV N110 is something I was waiting to buy for months before I finally got some. I think VV pushed a small batch, and that's it. I sit here and watch, every day, nearly all day, what hits the retailers and what doesn't.