khadlja:

if by best friend you mean someone who loves you fiercely; supports you through the thick and thin; is always there for you; watches out for you; constantly showers gifts and blessings upon you; forgives you over and over and over; makes sure you’re okay; helps you grow; never abandons you; only wants the best for you; and understands you in ways you don’t even understand yourself, then yes, Allah (swt) can be and is your best friend.

وَاللَّهُ وَلِيُّ الْمُؤْمِنِينَ - and Allah (swt) is the close and intimate friend of the believers. [3:68]