And why do you not spend in the cause of Allah while to Allah belongs the heritage of the heavens and the earth? Not equal among you are those who spent before the conquest [of Makkah] and fought [and those who did so after it]. Those are greater in degree than they who spent afterwards and fought. But to all Allah has promised the best [reward]. And Allah , with what you do, is Acquainted.
— Quran, sura 57 (Al-Hadid), ayah 10
Hazrat, I wanted to confirm when Allah says “promised” them the best reward means even the Sahaba who accepted Islam after the Conquest of Makkah are promised Jannah?
بسم الله الرحمن الرحيم
Allah ﷻ mentions in the verse you’ve quoted;
و كلَّا وَعَدَ اللَّهُ الْحُسْنَى
And every one of them (i.e. the Sahabah, may Allahﷻ be pleased with them), Allah ﷻ has promised the best [reward].
The word al-Husna, translated as the best, refers to Jannah. Allah ﷻ has promised Jannah to all those who accepted Islam, fought and spent in His ﷻ way before the conquest of Makkah as well as all those who accepted Islam, fought and spent in His ﷻ way after the conquest of Makkah.
However, as for the former, their ranks in Jannah are far higher and superior than the latter. Allah ﷻ mentions earlier in the verse;
لَا يَسْتَوِي مِنكُم مَّنْ أَنفَقَ مِن قَبْلِ الْفَتْحِ وَقَاتَلَ ۚ أُولَٰئِكَ أَعْظَمُ دَرَجَةً مِّنَ الَّذِينَ أَنفَقُوا مِن بَعْدُ وَقَاتَلُوا ۚ
Those of you who spent and fought in the Cause of Allah before the victory (conquest of Makkah), shall receive higher ranks of honor than the others who spent and fought thereafter.
Allah ﷻ describes the former as gaining greater rewards/ranks (أعظم درجة) than the latter. The wording used is ism at-tafdheel i.e. the comparative degree. This wording is normally used for comparison between two things sharing the same characteristic expect that one possesses that characteristic to a greater degree than the other. Regarding the above verse, both groups share in the great rewards (Jannah) promised by Allah ﷻ except that the magnitude of rewards granted to the former is greater than the latter.
In conclusion, Allah ﷻ has promised Jannah for all the Sahabah, those who spent and fought before and after the conquest of Makkah except their ranks in Jannah varies i.e. the former will enjoy higher and loftier ranks than the latter.
Similar to this verse, Allah ﷻ mentions;
لاَّ يَسْتَوِى الْقَـعِدُونَ مِنَ الْمُؤْمِنِينَ غَيْرُ أُوْلِى الضَّرَرِ وَالْمُجَـهِدُونَ فِى سَبِيلِ اللَّهِ بِأَمْوَلِهِمْ وَأَنفُسِهِمْ فَضَّلَ اللَّهُ الْمُجَـهِدِينَ بِأَمْوَلِهِمْ وَأَنفُسِهِمْ عَلَى الْقَـعِدِينَ دَرَجَةً وَكُـلاًّ وَعَدَ اللَّهُ الْحُسْنَى وَفَضَّلَ اللَّهُ الْمُجَـهِدِينَ عَلَى الْقَـعِدِينَ أَجْراً عَظِيمًا
Those (able-bodied) believers, who sit (at home) without having any (valid) disabilities are not equal (in rewards and ranks) to those (believers) who strive hard and fight in the cause of Allah with their wealth and their lives. Allah ﷻ has preferred in grades and ranks, those who strive hard and fight with their wealth and their lives above those who sit (at home). Unto each, Allah ﷻ has promised the best [Jannah], but Allah ﷻ has preferred, by great rewards, those who strive hard and fight (above those who sit at home). (4:95)
In the above verse, both groups of believers, Allah ﷻ has promised for them Jannah, except that one group will enjoy greater rewards and higher ranks in Jannah over the other.
And Allah Ta’āla Knows Best
Mufti Kaleem Muhammad
Darul Iftaa, Jaamia Madinatul Uloom (Trinidad)
 فإن اسم التفضيل يدل على المشاركة فيما اشتق منه اسم التفضيل وزيادة من أخبر عنه باسم التفضيل في الوصف المشتق منه ، أي فكلا الفريقين له درجة عظيمة
This answer was collected from Fatwa-tt.com, which is operated by the Darul Iftaa of Jaamia Madinatul Uloom (Trinidad and Tobago) under the advice and guidance of Mufti Ebrahim Desai (Daamat Barakaatuhum) of South Africa.