When it comes to major expansions, here are some things to consider:
None of them are particularly rare because they were the main source of expanding material for most people and were printed in highest quantities compared to mini-expansions and promo tiles.
The expansions that came first chronologically, had more opportunities to be reprinted and/or included in bundles/special editions, than the younger expansions. So the expansions with higher quantities are poised to be less rare from the supply perspective.
After the switch to C2, all of the expansions were eventually reprinted in the new artwork, but in the meantime between the reprint and C2 switch, C1 was the only source of specific expansions at any given time, meaning expansions reprinted later would have their C1 stocks depleted more than expansions reprinted sooner. Considering these were mostly expansions which already had smaller print runs, their overall quantities were dwindled even further making them more rare in C1 form.
Lastly the sole expansion which didn't see a C2 reprint (and never will) is the Catapult (exp.7). Anyone wanting to buy it is required to dig into the C1 stocks and this has led to a massive price inflation in the recent years, making the once unwanted box the rarest C1 major expansion today.
So to answer your question you could make an imaginary plot showing the timeline on x axis and rarity on y axis. Expansions 1-6 and 8-9 would form a continuous increasing trend in rarity, and expansion 7 would be above it as an outlier.