Why does the Torah add the word "le'Soldosam" by Levi exclusively?
Oznayim la'Torah: Because although the Torah only lists the fathers of Reuven and Shimon, 3 it lists also the children of the fathers of Levi.
Seforno: As opposed to Reuven and Shimon, where it only lists the children who went down to Egypt together with them, all of whom had already died - as the as the Pasuk wrote in Sh'mos, 1:6.
Oznayim la'Torah: As implied in the words "Mishpachas Reuven" and "Mishpachas Shimon".
Why does the Torah single out Levi, and subsequently K'has (in Pasuk 18), Amram (Pasuk 20) Aharon (Pasuk 23) and Elazar (Pasuk 25) for special mention?
Ramban: a. in honor of Moshe and Aharon, and b. because in their own right they were distinguished pious men. Also refer to 6:16:2:1.
Seforno: Because Levi, who lived longer than all his brothers, and subsequently K'has and Amram, taught their children 'Torah' to the extent that Moshe, Aharon and Miriam came from them; Aharon, who went on to marry the sister of the esteemed Nachshon, fathered leaders of the generation, 1 who would later be chosen as Kohanim, and Elazar took as a wife a daughter of Puti'el, 2 also an esteemed leader of that generation, and fathered Pinchas, who merited a covenant of peace.
Rashbam: In presenting the entire chronology of the world from Adam through to the second Beis-ha'Mikdash (inclusive), the Torah lists the years from Adam to No'ach and from No'ach to Avraham (in detail); then it lists the years of Avraham, Yitzchak, Ya'akov, Levi, K'has, Amram, Moshe, Yehoshua and the Shoftim; the kings, the seventy years of Galus Bavel and the years of the second Beis-ha'Mikdash (in Daniel).
Targum Yonasan: The Torah mentions them because each one merited to have children who were Tzadikim and lived a long time until they saw righteous offspring - Levi lived to see Moshe and Aharon; 3 K'has lived to see Pinchas (alias Eliyahu Kohen Gadol), who will take Yisrael out of Galus in the time of Mashi'ach; Amram, who married his righteous aunt Yocheved, besides fathering Aharon and Moshe, lived to see the sons of Rechavya, the son of Gershom ben Moshe, and Aharon, who married Elisheva bas Aminadav, the sister of Nachshon, who bore him Nadav and Avihu, Elazar and Isamar.