I go to Panama City Beach twice a year with my 5'er. If you are going to Panama City I would agree with 231, but if the Beach 231 brings you in on the wrong side of the Hathaway bridge and you have a bunch of traffic to get to the Beach side.
Go south on I65 to exit 114 and turn left on hwy 60. After a little over a mile the road makes a wide highway speed turn to the right and you are now on hwy 55. Stay on hwy 55 all the way to Florala, AL where you turn right on hwy 331. Stay on 331 past I10 and turn left on hwy 20 (331 south of 20 across the bay is under construction). Stay on 20 to hwy 79 where you will turn right to US98. Turn left on 98 to Panama City Beach.
All of this route after you leave I65 is either very nice two lane or 4 lane but non limited access. Other than Andalusia and you go around it there are no big towns and I have always had little traffic on this route.
Enjoy.
