Destin, FL, North Myrtle Beach, SC or Panama City, FL?

I personally love Destin or really any of the beaches in that area. I've also been to Pensacola. But I NEVER travel in the summertime. I have always gone in April and there are no crowds, spring break is over and it's just lovely and peaceful. Also, I have stayed at the Army MWR site there, which was tucked away from the main bustle of Destin.
North Myrtle Beach is great too, but I preferred more in my pre-child days. :) There are some beautiful places there, once I can afford to fly that way, I probably will give it another try.
If you go between June-August, any beach town will be busy and crowded. Even if you travel during the week. If you could, I would personally advise going in August because there are no holidays, long weekends, etc and towards the middle and end of the month people are starting to head home to get ready for school. But if you could postpone that long, then you might find it less busy.

We LOVE the Hilton Golf Resort and Spa at SanDestin. We spent many Spring Breaks there over the years.

LOVE the beaches, food, things to do. That hotel is inside a gated along with some condos as well. Nice safe place to vacation.

I've never been any other time than Spring Break so I can't answer to how the weather is at the time of year you plan to go.

We went to Myrtle Beach a couple of years ago and my husband decided we should go back next month. Even though we were just there a couple of years ago I honestly don't remember too much about it. I remember thinking it was way too hot and humid and a little too touristy (some of it in a tacky tourist sort of way--and I lived in the land of tacky tourist areas in South Dakota for several years and loved those places!). We had fun, but I can't say I'm super excited to go back. Personally I would prefer going someplace a little more subdued. I've never been to the other places you mentioned so I can't compare.

I have to second it....NMB!

Destin is insanely busy during the summer, but has much more to offer than Panama City. If you want to have options other than the beach (water parks, aquarium, go carts, mini golf, etc), I'd go there. If you are primarily beach people, try Blue Mtn Beach, Greyton Beach, or San Destin. There are smaller beaches about 20 minutes east of Destin. When the real estate boom in the area started, they refused to allow condos higher than 3 stories. So unlike Destin, which has hundreds of people staying in a single condominium complex all sharing the same strip of private beach, you get a much smaller building and have much more beach to yourself. We go in late Sept/early Oct, pricing is off season so it's significantly cheaper, but the weather is still great.

Look into Indian Pass, Florida. Off the beaten path, not touristy. That is where we are looking to book a house next summer!

I have never been to North Myrtle beach, but Destin and Panama are WAY too touristy for my liking. The beaches are packed. High rises everywhere. I have lots of friends who LOVE that atmosphere, but personally, I don't care for it. The beaches there are lovely, but we like to go to less traveled Florida beaches. Cape Sandblast was a wonderful trip. We rented a house right on the beach. Shared the shore with maybe a dozen or so people. Saw lots of wildlife including dolphins, sea turtles, and sharks.