I remember learning that we cannot know which commandments are more important than which others -- God gave them all to us and we don't say that we prioritize some over others (aside from the special priority to save a person's life). Avot 2:1 touches on this, saying to be as careful with a light commandment as with a grave one because we don't know their rewards. And I know we're not supposed to interrupt the performance of a mitzvah to do a different mitzvah, so we don't apply priorities there.
Yoma 8:8 teaches:
Repentance effects atonement for light transgressions: [the transgression of] positive commandments and negative commandments. And for severer transgressions [repentance] suspends [the divine punishment], until Yom HaKippurim arrives and effects atonement.
Is the mishna speaking in the abstract, saying that for some unspecified transgressions you can atone immediately and for others you have to wait until Yom Kippur? Or, when it says "light transgressions" (עֲבֵרוֹת קַלּוֹת), are we supposed to understand which those are? I understand the passage in Avot to be saying "commandments you think are light (or grave)", but maybe that's a misunderstanding?