We are on GSD puppy #4 and this is what we have always gone with.
Puppy 1, 2, and 3 were with us in Apartment Complexes with lots of dog friendly neighbors. Without a backyard you had to take the pup on a walk and it would encounter other dogs, places where other dogs have been, etc. It was always fine.
We stayed away from Parks and Pet Stores for a couple of reasons. Generally Parks have a LOT of dogs in a very concentrated area, sharing water and disease. I avoided Pet Stores that had shelter dogs for adoptions on the weekends. I applaud places like PetsMart and PetCo for allowing rescues the venue to showcase their adoptables...but we all know shelters are also rife with disease and while no one would intentionally bring a sick dog there are incubation periods and it does happen.
I do go to smaller holistic pet stores or places that are not really busy and do not have large animals. Stores like Lowe's are great, Outdoor flea markets, Tractor Supply. College Campuses are AWESOME.
Puppies 2, 3, and 4 went to SchH training once a week to see good dog people and other safe dogs. Restaurants where we could sit outside on the sidewalk as people go by.
Basically, do not underestimate the power of the "Walk". Walking in your neighborhood at a busy time of day can let your pup encounter cars, people working on their lawn (mowers and blowers are great noises!), people on bikes, people with other dogs, garage doors going up, kids, strollers, flags, maybe livestock if you live near the country...anywhere new that you can walk your dog is a good place to go.
And remember that socialization is NOT necessarily about interaction. You don't have to let every person pet your new puppy and you do not have to let every dog smell it either. (Too much can actually be overwhelming...think about people...after a little while you just DON't want to be poked and prodded anymore)
It's about Exposure and learning what the world looks like and smells like and how to act in it.
I brought Katie home from the shelter when she was 5 months old. Unfortunately I didn't get to raise her from day 1 til now, but I'm glad the previous owner did a good job on socializing her. I didn't take her out to meet other dogs until her 2nd round of shots were done. She was already potty trained, so she never went pooped or peed in the house. She also rarely poops or pees on her walks or in public.. I also discovered that Katie is friendly with ALL ages! When I bring her to the store, she loves children calling her "cute" and "pretty girl".
I've never brought Katie to a pet park because there's not that many around my area. The one around 20 minutes away from me has some horror stories, and a lot of drama regarding owners not caring about their dogs biting other dogs. To me, I don't want to stress myself out with dealing about that kind of stuff, so I just don't bring Katie there.
I live 4 minutes away from a Petco, so I usually bring Katie there. Every day of the week, there's a different number of people and dogs there. She interacts with the store's staff members, and sometimes other dogs with permission from the owner. A plus about Petco is that there's AC, and with the 95 degree weather out, I walk Katie around the store for about an hour! Not the ideal place to walk a dog, but it's keeping us both cool, and giving us both some daily physical activity.
My petco also has a bi-weekly "puppy social" on Sundays. With proof of vaccinations, you can bring your puppy to the puppy social where your puppy can interact with other puppies. The pet trainer first seperates the session by letting the "little" dogs play first (by weight class), then the bigger puppies such as the Huskies, GSDs get to play second. Then after that, all puppies interact and smell/play with each other. The owners also interact, telling each other various things such as common behavior issues, to what brand dog food is eaten. I think it's a good experience, and best of all it's free. The owner also gives out Halo dried treats, and Wellness Pet Jerkies, and various coupons such as coupons for Petco Brand dog toothpastes and toothbrushes.
I only Walk Katie in the evening due to the heat (around 8pm), and I walk her until 9pmish. A lot of people run/jog, and also walk their dogs around this time too. I take "new routes" around my neighborhood so Katie doesn't get to "used" to the surroundings and keep sniffing at familiar places. She loves watching people water the grass, and when she sees automatic lawn sprinklers on, she looks at me with these pleading eyes (She loves running through sprinklers). When Katie and I run into someone walking their dog, I ask for permission if they may sniff each other, and do so.
That's really all I do for "socializing" her, and I think it's good enough. Katie is a confident, friendly, and smart puppy. The only time she cowers is when I pick up my pots and pans (maybe her previous owner had something to do with that), but she is excited whenever she sees new people/dogs around.