When Megilah reading is Motza'ei Shabbos (Saturday Night) what is said first, Havdalah or Megilah and why?
Ashkenazim and Sefardim do Havdalah after Mikrah Megilah as per S.A. Rema 693 1 in the name of the Sefer Minhagim, the reason being that we do whatever we can to push off leaving Shabbos. Since there is this consideration we do not look at which is tadir. The frequency of a mitzvah is only considered as a factor for precedence when all other things are equal.
However Kaf HaHayim (Sefardi) does say in the name of Bigdei Yesha that if he is reading Megila in his home, better to make Havdalah first so as to minimize the chance of him doing Melacha before Havdalah.