I'm not going to really go too much into the whole why is one food better than another thing because I think everyone should do their own research. I did a lot when I got my GSD and what I found out surprised me. There is a lot more that goes in to choosing a good food for your dog other than price. And for those that are worried about price keep this in mind... look at the amount of food you're supposed to feed your dog each day. A good example is
Orijen 6 Fish. You look at that and might go OMG! $24-88/bag?! Yeah, that's expensive at first glance but guess what? You might end up feeding your dog half as much food per day so the bag lasts twice as long so it really doesn't cost any or much more per month.
In the case of my GSD I actually went from Purina Pro Plan Focus Large Breed Puppy because it was locally available at Tractor Supply and decent enough and then when he got to 6 months I decided it was time to switch him to a normal adult food. After researching a lot of different brands I decided to give Orijen a try. His energy level went up (not so sure that's a good thing with a GSD puppy lol), his coat got much shinier, and his poop firmed up. I also will occasionally throw some Pet Select on top of his dinner. Every dog will be different though. GSDs for example are notorious for having sensitive stomachs. Some just can't handle the protein in the Orijen and have constant "pudding butt" or diarrhea so people will use Acana instead (same manufacturer, lower protein content), others are fine with it.
Basically you want to think about several things:
Type of food - Dry kibble, canned food, raw (or "BARF") diet, or some combination of the above.
Price - obviously everyone is going to take this into consideration - what are you willing to spend on your dog?
Availability - do you have to get your food from your grocery store? Pet store? Specialty pet store? Online? I will say if you decide to buy online make sure you have enough food on hand or have a local backup plan just in case something goes wrong, like an ice storm while your food is in route. *cough*
Your dog - Ultimately they make the decision for you, you just have to know what to look for. Will the dog eat it? Does he scarf it down, nibble at it slowly, look at you like you're a moron and tell you to eat it yourself? In the case of my dog I actually feed him in a
Kong Wobbler most of the time just to slow him down and give him something to do, otherwise he inhales the food in 20 seconds.
Do you notice things like itching, lack of energy, upset stomach? Pay attention to the poop, it can actually tell you a lot.
Reviews from people that actually use the food you're looking at - be it at Dogfoodadvisor.com, Chewy.com, online forums, etc. You can also find out about recalls and stuff that may be common to a certain manufacturer, you'd be surprised what may end up in your dogs' food.
I could go on and on but you get the idea. Not all food is created equal. Not all dogs (or their stomachs) are the same. And just like everything else in life everyone has their opinion. You have to form your own.