- Idolatry (Some include any chilul Hashem-the defamation of Hashem's name-which is a very big category of mitzvot violations)
- Murder (Does not include self-defense. Note that killing does not necessarily equal murder. Yay legalese!)
- Incest and Adultery (Also known as "forbidden relationships," but that can also be a much larger category)
Why these mitzvot and not others? The idea of pikuach nefesh comes from Leviticus/Vayikra 18:5, "You shall therefore keep My statutes, and My ordinances, which if a man do, he shall live by them: I am HaShem." From this psuk (verse), the rabbis concluded that "That he shall live by them, and not that he shall die by them." But if that is the case, why shouldn't we be able to violate any mitzvah for the purpose of saving our lives or the life of another?
I don't know the answer to that, at least not the "official" answer. My guess is that each of the exceptions listed above denies Hashem and His creation. Unjustified murder is acting like a Gd, doling out life and death. Incest and adultery deny the honor of the relationships closest to us, demeaning them as a person and family member. Idolatry is pretty self-explanatory. I feel like I've heard this idea before, but I don't remember where.
But playing devil's advocate, the rabbis say that violating the laws of Shabbat are a desecration of Hashem's name (wow, that's redundant) because violating Shabbat is denying that Hashem created the world in 6 days and rested on the 7th. Given my reasoning above, it seems like desecrating the Sabbath should also be an exception. Going even further, if Hashem really gave us mandatory oral and written law at Sinai, then violating any mitzvah would be denying that Hashem created humanity/the world and chose Israel as His nation. In that case, it would be prohibited to violate any mitzvah even to save a life. Where would we draw the line?