Best fit allocates the process to a partition which is the smallest sufficient partition among the free available partitions. 
Implementation:
1- Input memory blocks and processes with sizes.
2- Initialize all memory blocks as free.
3- Start by picking each process and find the
  minimum block size that can be assigned to
  current process i.e., find min(bockSize[1], 
  blockSize[2],.....blockSize[n]) > 
  processSize[current], if found then assign 
  it to the current process.
5- If not then leave that process and keep checking
  the further processes.

Best Fit algorithm in Memory Management : https://www.geeksforgeeks.org/program-best-fit-algorithm-memory-management/