Gemara

R. Yosi bar Chanina: "Mishpachah u'Mishpachah" teaches that Kohanim and Leviyim abandon Avodah in the Mikdash to go to hear the Megilah.

Rav Yehudah (and a Beraisa): Kohanim, Leviyim and Yisraelim abandon their Avodah, singing and Ma'amados (overseeing the Avodah) to hear the Megilah.

In Rebbi's house they abandoned learning Torah to hear the Megilah. They learned from a Kal va'Chomer:

We abandon Avodah to hear the Megilah. All the more so we abandon Torah to hear the Megilah!

Question: The Kal va'Chomer assumes that Avodah is greater than learning Torah, but this is not so!

R. Shmuel bar Uniya: The angel that came to rebuke Yehoshua came for Bitul Torah, not for Bitul Avodah. This shows that Torah is greater than Avodah.

Answer: Torah of the Tzibur is greater than Avodah, but an individual's Torah is not.

Rava: We learned that reading the Megilah has precedence over Avodah (above, 2) and over learning Torah (3:i);

A Beraisa teaches that a Mes Mitzvah (burying an unattended corpse) has precedence over learning Torah:

Beraisa: We abandon Torah for burial and to bring a bride to the Chupah.

A Beraisa teaches that a Mes Mitzvah takes precedence over Avodah:

Beraisa - Suggestion: If one was heading to offer Korban Pesach or circumcise his son, and heard that a relative died, perhaps he interrupts to bury the Mes (even though he will lose the Mitzvah he was engaged in)!

Rejection: "Lo Yitama" (just like a Nazir does not become Tamei for a relative, also someone doing a Mitzvah).

Suggestion: Perhaps just like he does not interrupt for relatives, he does not interrupt for a Mes Mitzvah!

Rejection: "Ul'Achoso" - he does not interrupt for his sister; but he interrupts for a Mes Mitzvah.

Question (Rava): Which has precedence, reading the Megilah or a Mes Mitzvah?

Perhaps reading the Megilah comes first because it publicizes the miracle;

Perhaps a Mes Mitzvah comes first because of Kavod ha'Briyos.

Answer (Rava): The Mes Mitzvah has precedence:

Kavod ha'Briyos is such a great Mitzvah that it overrides a Lav of the Torah.

Rishonim

The Rif and Rosh (1:3) cite the Gemara's proofs that reading the Megilah overrides Avodah and Torah, and that a Mes Mitzvah overrides all of these because of Kavod ha'Briyos.

Ran (DH Talmud Torah and DH Mes): The Gemara says that we Mevatel Torah for any Mes, not only for a Mes Mitzvah. It mentions a Mes Mitzvah for similarity to Avodas Mitzvah. Alternatively, it is Reshus to Mevatel Torah for a Mes that is not a Mes Mitzvah, but it is a Chiyuv to Mevatel for a Mes Mitzvah. Kavod ha'Briyos overrides a Lav of the Torah, i.e. Isurim mid'Rabanan, which are based on "Lo Sasur".

Rambam (Hilchos Megilah 1:1): Even Kohanim Mevatel Avodah to come to hear the Megilah. Similarly, we Mevatel Torah to hear the Megilah; a Kal va'Chomer teaches that we Mevatel other Mitzvos for this. The only Mitzvah not overridden is a Mes Mitzvah with no one to bury it. One who encounters a Mes Mitzvah buries it and then reads.

Question: We learned that Megilah overrides Torah from a Kal va'Chomer from Avodah. Since Avodah does not override Talmud Torah of the Tzibur, there is no source that Megilah overrides Torah of the Tzibur. Why didn't the Rambam specify that it overrides only Torah of individuals?

Answer (Lechem Mishneh): We know that a Mes Mitzvah overrides Talmud Torah of the Tzibur, yet the Gemara asked whether or not a Mes Mitzvah overrides Megilah. We must say that also Megilah overrides Torah of the Tzibur.

Poskim

Shulchan Aruch (OC 687:2): We Mevatel Talmud Torah to hear the Megilah. A Kal va'Chomer teaches that we Mevatel other Mitzvos mid'Oraisa for this.

Source (Gra DH Kal): Pe'ah (1:1) and Kidushin (39b) equate Torah to all the Mitzvos.

Kaf ha'Chayim (11): Even a group learning together interrupts for Megilah. We learn from Beis Rebbi. Surely, there were many people learning together there. The Magen Avraham suggests that Megilah does not override Talmud Torah of the Tzibur, i.e. if all of Yisrael were learning together.

Question: Why is this called Bitul Talmud Torah? Reading the Megilah is also Torah!

Answer #1 (Ran 3a DH Amar): One should leave where he is learning to hear the Megilah in the Beis ha'Keneses with a large Tzibur.

Answer #2 (Teshuvas Beis She'arim OC 368, Hagahos b'Sof): Merely listening to the Megilah is considered Bitul Torah if one could be saying Divrei Torah out loud and contemplating them.

Shulchan Aruch (ibid.): The only Mitzvah not overridden is a Mes Mitzvah lacking (Rema - the necessary amount of) people to bury it. One who finds a Mes Mitzvah buries it and then reads.

Magen Avraham (4): We do not override Avodah to bury relatives or any other Mes with people to bury it, all the more so we do not override Megilah for such a Mes! Tosfos (Shevu'os 30b DH Aval) explains that only a Mes Mitzvah overrides Pesach and Milah because it is a great disgrace when there is no one to bury the body. However, burial overrides Talmud Torah even if there are others to bury it.

Mishnah Berurah (8): If there are others to bury a Mes we do not override Mikra Megilah unless the Mes is a Chacham, or if they already began taking it to burial.

Kaf ha'Chayim (14): Ten people are considered necessary for burial, in order to say Kaddish and Birkas Avelim.

Rema: This is only if there is time for both Mitzvos. If not, we do not Mevatel any Mitzvah mid'Oraisa for the Megilah.

Beis Yosef (DH v'Tzarich): The Ran says this explicitly. Also the Rambam writes that one who encounters a Mes Mitzvah buries it and then reads. Surely, these words teach that a Mes Mitzvah comes first only if he will have time to read later. Tosfos (DH Mevatlim) explains that the Chiyuv Avodah came (first) at dawn. We read the Megilah first only because it is a better Pirsum ha'Nes to read with the Tzibur.

Rebuttal (Taz 2): The Gemara says that a Mes Mitzvah overrides Megilah because Kavod ha'Briyos overrides a Lav. This is like Rashi explains; a Chacham may ignore a lost object if it is beneath his dignity to carry such an item in public. The Lav "Lo Suchal Lehis'alem" is totally uprooted. The same applies to a Mes Mitzvah. It has precedence even if the other Mitzvah will not be fulfilled, all the more so regarding Megilah which is only mid'Rabanan. The Rambam writes that one buries the Mes Mitzvah and then reads to teach that the Mes comes first even though one loses the ideal Pirsum ha'Nes of reading with the Tzibur. The Rambam does not discuss when there will not be time to read later. Surely, if one encounters a Mes in the field (and has no Megilah there) he should not leave it until after he reads. Perhaps dogs will eat it! The Rambam connotes that Megilah overrides Avodah just like it overrides Torah, i.e. even if there is not time for both. There is no connotation that Tosfos disagrees. The Ran explicitly says that Megilah comes first only if there will be time for the Avodah later; this does not seem correct.

Question: Reading the Megilah is only mid'Rabanan. How can it override Mitzvos mid'Oraisa?

Answer (Taz 2): Mikra Megilah is from Divrei Kabalah (Kesuvim), which is like Divrei Torah.

Magen Avraham (5): Similarly, we do not override Milah if there will not be time. However, Terumas ha'Deshen says oppositely. I.e. if there is time for both, Milah is first; if not, we only read the Megilah. The reason is because the Milah can be done the next day.

Gra (DH Aval): Because Ner Chanukah publicizes a Nes, it overrides wine for Kidush (Shabbos 22b). If Kidush over wine is mid'Oraisa, Mikra Megilah overrides Mitzvos mid'Oraisa for the same reason. The Rema must hold like the opinion (in Tosfos Pesachim 106a DH Zochrehu) that Kidush over wine is only mid'Rabanan.

Rema: A Mes Mitzvah overrides Megilah only if there will be time to read afterwards.

Taz (2): The Rema contradicts himself. Above he said that if there is not time for both Mitzvos we do not Mevatel any Mitzvah mid'Oraisa for the Megilah.

Rebuttal (of Rema - Magen Avraham 6): According to the Rema, if it is almost night Mikra Megilah overrides a Mes Mitzvah if the Mes can be buried at night. This is difficult. If Mikra Megilah does not override Mitzvos mid'Oraisa, a Mes Mitzvah should override Mikra Megilah, just like it overrides Mitzvos!