Of course, I'm not calling anyone inferior or superior, just clarifying what might be a touchy fact or two that some opinions seem to reject. Though I do believe that those souls chosen to be incarnated as Cohens and Levites may be "superior" in the Heavenly ranking, but that's for another subject.
Again, this comes back to the the taints on the soul and Tikkun Olam. You see, when all the original souls lived, back when the Hebrews left Egypt, each and every Hebrew at the time ended up worshiping the Golden Calf. The Levites and Kohens, However, were the only ones who didn't.
God appointed Aharon to be the Kohen Gadol of the time, and therefore making all his descendants Kohanim.
Until the transgression of the Golden Calf, the firstborn sons were the ones who were designated to do the holy service in the Mishkan, the Portable Sanctuary, in the Desert. Since no one from the Tribe of Levi worshipped the Golden Calf, the Almighty replaced the firstborn with the Tribe of Levi to serve in the sanctuary (Numbers 3:11-12).
The reason why they are considered superior, in some sort, is because they didn't commit that initial sin which started the whole Tikkun Olam concept.
But just because they have that merit, doesn't actually make them superior to the rest. Moshe Rabeinu was not a Kohen (EDIT: Thanks Harmonious for the correction). However the bible clearly states :"And there hath not risen a prophet since in Israel like unto Moshe"(Devarim 34, 10). It also says that we each have the potential to achieve our limits like Moshe did.
This shows us that the average farmer could become the greatest of all men, regardless of whether he is Kohen, Levite, or Israelite.