Which Haftarah is read with the least frequency?
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Going through all of the possibilities (already mentioned, and that I can think of) mathematically, using the data on frequency of year types provided on Remy Landau's page, and the tables of kevios in Jewish Chrononomy, by Yehudah (Leo) Levi:
Any others that I've missed?
So far it looks like the rarest one would be Tzav's, assuming Jerusalem and the minhag to always read the special haftarah for Shabbos Hagadol. Otherwise, the rarest haftarah would be Miketz's.
Considering that the questioner is asking in Kislev, I'd bet it's the haftarah for the second Shabbat of Chanukah.
I believe Tazria alone (the story of Na'aman) is quite rare; very often it's either combined with Metzora (and we read the subsequent story of the four lepers), or some special week (HaGadol, HaChodesh, or the like).
Kedoshim's alone ("go tell Jerusalem about all its abominable acts") is also quite rare; when there's combined Acharei-Kedoshim, many communities break the usual rule (to use the second parsha's haftorah) and instead read that of Acharei. Even when separate, often Kedoshim winds up on some other special haftorah, and in the rare event that it wouldn't, Rabbi Soloveichik is quoted as saying you read the Acharei haftorah two weeks in a row! (Which I suppose makes it a "closet Haftorah.") We don't like going around talking about Jerusalem's abominations ...
I suspect it may be Tzav's, since Tzav usually coincides with the week of one of the many special haftaros that we read around that time of year. (I've also heard that the most rarely read haftara is Tzav's, but I don't remember hearing it from a reliable source.)
If it is Tzav's, that's pretty ironic, since Tzav's is one of the few haftaros mentioned in the g'mara.
A computer analysis of the 247 years and the haftaros read in each year will quickly yield an accurate result, however.
Shalom's 2nd answer, Kedoshim, is the correct answer according to the minhag that most Ashkenaz minyanim follow, and appears to be the case with the Sefardic custom too except that it is a different haftarah that is rarely read for them.
The prevailing Ashkenzaic custom is to always recite the portion from Amos on at least one week. Yechezkel, Chapter 22, is listed as the haftarah for "kedoshim".
However when Acharei and Kedoshim are recited together, the haftarah for Acharei (Amos) is read. This counts for all 12-month years.
Now let's look at 13 month years: - If Pesach runs Shabbat to Shabbat (like it did this year) then the Shabbat after Pesach will be Acharei and the one after that will be Kedoshim, and dependent on whether you are in Israel or not, the first or second of these will be 29th Nisan and you will read Machar Chodesh. On the other one, regardless of the sedra, you read from Amos. (In my community we are outside Israel and read Amos with Kedoshim).
That leaves only one type of year. When Rosh Hashanah was a Shabbat and the year is deficient so Pesach begins on a Tuesday. Then you read Acharei on 26th Nisan and Kedoshim on 3rd Iyar with the haftara from Yechezkel.
That ends up about 5.8% of the time using the Remy Landau's chart (see other answer). In the last 50 years it has been read 3 times, then last time was 5757 (1997) and the next time will be 5784 (2024).