if you need only one best hit for each query, use -num_alignments and -num_descriptions flag as fallows.
Usage:
blastn -query fasta.file -db database_name -outfmt 6 -num_alignments 1 -num_descriptions 1 -out output_file
Example:
blastn -query fasta.file -db nr -outfmt 6 -num_alignments 1 -num_descriptions 1 -out haktan.txt -dust no -task blastn
Output:
tabular form of blast output written in haktan.txt file
Enjoy!