"And You are the best of the merciful," meaning: the most excellent of those who show mercy. Allah may place mercy in the heart of whomsoever He wills, and that is from the mercy of Allah.
﴿وَأَنت خير الرَّاحِمِينَ﴾ يَعْنِي: أَفْضَلُ مَنْ رَحِمْ، وَقَدْ يَجْعَلُ اللَّهُ الرَّحْمَةَ فِي قَلْبِ مَنْ يَشَاءُ؛ وَذَلِكَ مِنْ رَحْمَةِ الله.