To me, about the most important and first command is "sit" because it will work in so many different circumstances if the dog executes it pretty reliably on command. I also like the hand signal for sit to be ingrained in the dog. With "sit" accomplished, you can stop a running or chasing dog, a dog about to walk into traffic, or buy time to get to the dog to retrieve him even if he won't "come" when you call. You can also stop him from gobbling his food or snatching food from a toddler's hand...or even just get his attention. I have used it in all of these examples even though my dogs knew and responded well to other, more appropiate commands. "Sit" is also easy to 'belt out' in an urgent situation....even at the start of an attack. Later, "sit"...followed by "watch me" or "stay" or "come" or "no" or "leave it" or "give it" or just about anything else. I could never make the promise that it will work for every dog in every situation, but, to me, it is the ultimate all-purpose command.
To illustrate, Brody was 14 months old but not trained at all when we first got him and I am still working on training him and Mia. But, the first command I started with was the "sit" command. Since we had not had him long, and he was not bonded well at the time, he would take off running and no way could we catch him. He knew no commands except had gotten pretty good with the "sit" we were working on, so when he would start to do something he shouldn't...like play the "catch me if you can 'cuz I'm the Gingerbread Man" game, I would give him a "sit" command to alter the course of his behavior until I could make him 'safe' again. He would sit and I would slowly walk over and pick him up...giving a "stay" hand signal. DH, on the other hand, would just run around behind him playing Gingerbread...to Brody's great delight! I recommended to DH that he not give chase...just issue a firm "sit" command, wait until he sat down, give it a second to show the game is over, then do whatever he needed/wanted to do. We rarely play Gingerbread outside anymore...unless someone forgets to 'holler' SIT.
I want all my dogs to have proper training, but with an adult dog, you definitely have to start somewhere and one command at a time is as good a way to start as any. With a pup, "sit" is always my first lesson. In Brody's case, first, he needed to learn what learning was. "Sit" and a few cubes of cheese taught him not only to sit on command but that learning 'tricks' was fun...and he has been excited to learn ever since.
Good luck with your endeavor. I am sure you will figure out what works best for you. These are just my experiences and opinions!