When looking for the dog park that's best for you, consider proximity, popularity, environment, rules, and patrons. The last thing you want is to drive an hour to a dog park that's overcrowded with jerks and muddy with feces everywhere and the never-ending stale smell of urine soaked into the ground.
A big dog park isn't necessarily a good dog park. I've found that my favorite dog park to go to is a simple fenced in area at a park in the middle of nowhere. Nobody knows about it, it's not advertised, and there's almost never any other dogs there. I have three large dogs that like to play rough, and it's best for me not to take them to a popular dog park because they get flustered and become protective over one another.
Driving an hour to go to a dog park might be necessary for someone who lives in an area that has little to no dog parks, but if you live in a more highly-populated area, it's worth checking into parks that are closer in proximity to where you live. If the dog park is an hour away, you're not likely to go very frequently. There are often times dog parks closer than you think, but they might take a bit of digging to find.
Do you enjoy bathing your dog? If not, look for a dog park that utilizes sand or pea gravel instead of dirt. Especially if the dog park is popular, as the more dogs running and peeing on grass, the less grass there will be and the more it will stink like urine. Some dog parks have hills and creeks throughout, which can make things more fun for your dog.
Also, pay attention to the patrons at the dog park. If you pull into the parking lot and see several people on cell phones who aren't paying attention to their dogs, they're likely irresponsible pet owners. If there's feces everywhere, it's likely they don't care to follow any rules. If there aren't any rules posted, you may want to find somewhere else to go. It's important that dog parks have rules posted for people to follow, and it's equally important that the patrons of the dog park follow those rules. These rules help prevent frustration, injury, and in some cases death. Aggressive dogs should at the very least be kept on a leash at all times. If a dog is aggressive, the only reason to even have it at a dog park would be to help it grow more comfortable around other dogs. Trainers may use this as a tactic; but in most cases, an aggressive dog should not be at a dog park.
So do your research, keep in mind the factors to look for, and once you find the right place, have fun! Engage with your dog, play fetch, encourage interaction, but pay close attention and prepare to physically intervene if anything negative occurs regarding your dog or the dogs around yours. The dog park can be a great place for you and your dog, or it can be the worst if you don't choose wisely and act according to the rules.