Superiority of Fajr and Asr Prayers
76) Narrated Abu Huraira that Allah' s Messenger said: Some angels come to you in succession by night and some others come to you by day. they meet together at the time of Dawn (Fajr) and Afternoon (Asr) prayers. Then those who passed the night with you, ascend to heaven. There Allah asks them though He knows best about them: In what state did you leave My slaves? The angels reply: When we left them, they were offering prayers and when we reached them, they were offering prayers.
(This Hadfth is sound and reported by Bukhari and Muslim).
The most important prayers are the Dawn (Fajr) and Afternoon (Asr) ones. Perhaps because one has to fight tendency for sleep anc rest at these times. Asr prayer is mostly the mid-prayer Allah asserted in the Qur'fin: "Guard strictly the (five) prayers especially, the mid-prayer (Asr) .." (2:238)