Best fast after Ramadan is Muharram and the best prayer after the fard is the night prayer
MUSLIM 1163a:
from Abū Hurayrah —
The most excellent fast after Ramadan is God's month. al-Muharram, and the most excellent prayer after what is prescribed is prayer during the night.
مسلم ١١٦٣a:
عن ابو هريره —
قَالَ رَسُولُ اللَّهِ ﷺ أَفْضَلُ الصِّيَامِ بَعْدَ رَمَضَانَ شَهْرُ اللَّهِ الْمُحَرَّمُ وَأَفْضَلُ الصَّلاَةِ بَعْدَ الْفَرِيضَةِ صَلاَةُ اللَّيْلِ