(And those foremost will be foremost.) Is about the Prophets, peace be upon them, while As-Suddi said that they are the residents of the utmost highs (Ahl Al-`Illiyyin, in Paradise). The meaning of foremost is that they were foremost in performing the acts of righteousness just as Allaah commanded them,
﴿وَسَارِعُواْ إِلَى مَغْفِرَةٍ مِّن رَّبِّكُمْ وَجَنَّةٍ عَرْضُهَا السَّمَـوَتُ وَالاٌّرْضُ﴾
(And march forth in the way to forgiveness from your Lord, and for Paradise as wide as the heavens and the earth.)(3:133) and,
﴿سَابِقُواْ إِلَى مَغْفِرَةٍ مِّن رَّبِّكُمْ وَجَنَّةٍ عَرْضُهَا كَعَرْضِ السَّمَآءِ وَالاٌّرْضِ﴾
(Race with one another in hastening towards forgiveness from your Lord, and Paradise the width whereof is as the width of the heaven and the earth.)(57:21) Therefore, those who rush to obey Allaah in this life and are foremost in performing acts of righteousness, will be among the foremost believers honored in the Hereafter. Verily, the reward is according to the kind of deed, and as one does, so he is judged. So Allaah said:
﴿أُوْلَـئِكَ الْمُقَرَّبُونَ فِى جَنَّـتِ النَّعِيمِ ﴾
(These will be the nearest (to Allaah). In the Gardens of Delight.)
Source: Tafsir Ibn Kathir (Tafsir of Surat Al-Waqi'ah)