Why does the Torah add "Me'od" after the word "Tov"?
Ramban #1: It hints here at death or at the Yeitzer ha'Ra, both of which are necessary for the world to exist (since the bad is necessary to establish the good).
Ramban #2: It inserts "Me'od", because, in honor of man, who was created on this day, He gave the creation a special praise.
Rashbam and Seforno: Because, whereas before the Torah was speaking about the P'rat (the details), here it is referring to the K'lal (the entire creation), which Hashem found to be perfect.
Why the extra 'Hey" in the word "ha'Shishi"?
Rashi #1: To hint that the Torah would only continue to exist on condition that Yisrael would eventually accept the five Books of the Torah (for which purpose it was created). 1
Rashi #2: To hint that it would continue to exist only if Yisrael would accept the Torah on the sixth of Sivan (2448). Otherwise, it would revert to null and void.
Seforno: To stress that this was the first of all Fridays (Erev Shabbos) on which all mundane activities cease, and which are followed by Shabbos (the day of rest).
An alternative text states that, on the sixth of Sivan, when Yisrael accepted the Torah, the entire creation was finally established and it was as if the world was created then.