There is a major discussion regarding the proper text for the Beracha recited upon concluding the Megillah reading. The fact that we make a Beracha at all is already recorded in the Mishna as a custom. It has become the accepted custom to make the Beracha, and it is even referred to by the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) as a "precious Beracha."
The question is regarding the text. All agree that the Beracha reads "HaNifra Lanu MiSarenu"-The One who pays back those who oppress us. The Rambam (Megillah 1:3), Rabbenu Hannanel and others authorities include the word "HaEl"-God, before "HaNifra". This was the text found in the Siddur of the Rashash, as cited by the Kaf HaHaim (691:16).
However, Maran, in Siman 691, does not mention that word "El." His opinion is based on the Tur, who cites Rav Amram Gaon, who questions adding another name of Hashem to a Beracha that already refers to "Elohenu"-Our G-d. Hacham Ovadia answered this argument by citing other places in the Tefila where this is done. For example, in Baruch She 'Amar, the divine name 'HaEl" appears after "Elohenu." Also, in Birkat Hamazon, the fourth Beracha reads, "HaEl," after referring to "Elohenu."
Hacham Ovadia rules in accordance with the Rambam, that the word "HaEl" should be inserted. This is the way the Siddurim under his auspices were printed. However, it should be noted that the Hida (Rav Haim Yosef David Azulai, 1724-1807) rendered the text without "HaEl." This is important because his opinion is considered authoritative regarding variant texts. In fact, many Siddurim commonly used, such as "Kol Yaakob" and "Magen Abraham," omit the word "HaEl." Moreover, Hacham Baruch himself did not say the word "HaEl," as seen on the video clip of him reading the Megillah. The custom of the Kabbalists of Bet El was also to omit "HaEl," as well.
The proper practice is, in fact, to omit "HaEl" from the Beracha.
---------------
Maran brings the custom to say "Arur Haman, Baruch Mordehai, Arurim HaReshaim, Bruhim Hasadikim, V'Gam Harbona Zachur LaTov."-Cursed is Haman, Blessed is Mordehai, Cursed are the wicked, Blessed are the righteous, and Harbona is mentioned with praise. The question is asked why the cursing of Haman and the wicked precedes the blessing of Mordehai and the righteous. The Aruch Hashulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) answers that chronologically, Haman and the wicked dominated before the rise of Mordehai and the righteous.
Harbona is the one who advised Ahashverosh to hang Haman. The Hachamim teach that this was in fact, Eliyahu HaNavi. Therefore, he is referred to as "Zachur LaTov," in the same manner that Eliyahu is called Eliyahu HaNavi Zachur LaTov.
While some have the practice to repeat this phrase three times, the accepted custom is to recite it only once after the Megillah reading.
SUMMARY
The proper text for the Beracha after the Megillah is "HaNifra Lanu MiSarenu" without adding the divine name, "HaEl."
