With awk it is pretty fast:
awk 'NR == num_line' file
When this is true, the default behaviour of awk is performed: {print $0}.
If your file happens to be huge, you'd better exit after reading the required line. This way you save CPU time See time comparison at the end of the answer.
awk 'NR == num_line {print; exit}' file
If you want to give the line number from a bash variable you can use:
awk 'NR == n' n=$num file
awk -v n=$num 'NR == n' file # equivalent
See how much time is saved by using exit, specially if the line happens to be in the first part of the file:
# Let's create a 10M lines file
for ((i=0; i<100000; i++)); do echo "bla bla"; done > 100Klines
for ((i=0; i<100; i++)); do cat 100Klines; done > 10Mlines
$ time awk 'NR == 1234567 {print}' 10Mlines
bla bla
real 0m1.303s
user 0m1.246s
sys 0m0.042s
$ time awk 'NR == 1234567 {print; exit}' 10Mlines
bla bla
real 0m0.198s
user 0m0.178s
sys 0m0.013s
So the difference is 0.198s vs 1.303s, around 6x times faster.