Assalamu alaikum Brothers and Sisters,
That is a problem with some translations of the Quran. Assalat alwustaa means the middle prayer. I really don`t know where ''the best prayer'' came from. I couldn`t find any evidence for that. If anyone does, please inform us. We are here to learn.
But there are several authentic hadeeths about the excellence of Fajr and Asr prayers.
Excellence of Fajr and Asr Prayers
Abu Musa (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "He who The observes Al-Bardan (i.e., Fajr and `Asr prayers) will enter Jannah.''
[Al-Bukhari and Muslim].
Abu Zuhair `Umarah Ruwaibah (May Allah be pleased with him) reported: I heard the Messenger of Allah (PBUH) saying: "He who performs Salat (prayers) before the rising of the sun and before its setting, will not enter the Hell.''
[Muslim].
Jundub bin Sufyan (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "He who offers the dawn (Fajr) prayers will come under the Protection of Allah. O son of Adam! Beware, lest Allah should call you to account in any respect from (for withdrawing) His Protection.''
[Muslim].
Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "There are angels who take turns in visiting you by night and by day, and they all assemble at the dawn (Fajr) and the afternoon (`Asr) prayers. Those who have spent the night with you, ascend to the heaven and their Rubb, Who knows better about them, asks: `In what condition did you leave My slaves?' They reply: `We left them while they were performing Salat and we went to them while they were performing Salat.'''
[Al-Bukhari and Muslim].
Jarir bin `Abdullah Al-Bajali (May Allah be pleased with him) reported: We were sitting with the Messenger of Allah (PBUH) when he looked at the full moon and observed, "You will see your Rubb in the Hereafter as you see this moon having no difficulty in seeing it. So try your best to perform the prayers before the rising of the sun and that before its setting.''
[Al-Bukhari and Muslim].
Buraidahu (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "He who misses the `Asr Salat (deliberately), his deeds will be rendered nul and void.''
[Al-Bukhari].
The angels for the night come at the time of `Asr when the angels for the morning are present. This is how the angels of the two shifts assemble at this time. The angels of the shift of `Asr leave their duty in the morning, and the angels of the morning shift resume their duty when the pious persons are engaged in Fajr prayer. This is how the two groups assemble again at that time. Thus, when the angels come or go, the people who are punctual in their prayer are engaged in Fajr and `Asr. Almighty Allah knows everything but even then He asks the angels about his pious slaves so that the piousness of the believers and their merit and distinction become evident to them.