According to the Hanaabilah the most important and stressed upon prayer from among the Nawaafil is the Salaah al-kusuuf because he (sallallaahu alayhi wa sallam) did not ever leave it when the reason to pray it was present.
Then comes the Salaah al-Istisqaa' because congregation has been stipulated for it. As a result, it resembles the obligatory prayers.
Then in importance is the al-Taraawiih prayer because he (sallallaahu alayhi wa sallam) kept praying it and only left it when he (sallallaahu alayhi wa sallam) feared it might be made obligatory. But it resembles the obligatory prayer from the point of view that congregation for it is legislated.
Then in importance comes the Witr prayer because there are narrations in regards to it which do not exist in regards to the two Rak'ah Sunnah of fajr prayer.
Finally comes in importance the two Rak'ah Sunnah of fajr prayer.
Kuwaiti Encyclopedia of Fiqh
Volume 27 Page 291