Piety

 Surah 2 is the longest surah in the Quran, making up something like 12% of the entire material. As you know, there are 114 surahs, so for one to take up over ten percent of the entire mass of the text that's definitely saying something. Of course, so many of the later surahs are only a few lines long. There aren't too many important subjects related to the faith that don't show up in one form or another in al-Baqarah ("the Cow"). Here is one of my favorite passage, relating to the concept of piety.

"It is not piety to turn your faces toward the east and the west. Rather, piety is he who believes in God, the Last Day, the angels, the Book, and the prophets; and who gives wealth, despite loving it, to kinsfolk, orphans, the indigent, the traveler, beggars, and for [the ransom] of slaves; and performs the prayer and gives the alms; and those who fulfill their oaths, when they pledge them, and those who are patient in misfortunate, hardships, and moments of peril. It is they who are the sincere, and it is they who are the reverent." 2:177

As Seyyed Hossein Nasr adds in his commentary, "Piety is understood as the obedience to God that is well established in the heart or as the sum of acts of obedience and devotion that led us closer to God." I'm always drawn more to the esoteric than the exoteric, and also to what someone does as compared to how they are performing for others, so there is so much about this passage that speaks to me.