There are certain commands that all dogs are supposed to learn, like 'sit' and 'heel'. If you go to a basic obedience class, these are the ones that will be taught. I like to teach my dogs hand signals as well as the verbal command, and in some cases they seem to respond more to the hand signal than the spoken word. Even within these basic commands, it's not like there is a specific set that every dog knows, and one person's exact command will be different than another ('down' vs 'lie down', for example).
Then there are commands that are more specialist. For example, a sheepdog will learn basic commands like 'come bye' and 'away'. As far as I know, come bye means to go left or circle the flock clockwise, whereas away means to go to the right or circle in an anti-clockwise direction. A sled dog will learn 'gee' and 'haw' for right and left, 'easy' to slow down, and so on. Show dogs will learn commands and routines specific to their acts, and many people will teach their pets a 'bang bang' command, accompanied by pointing two fingers at the dog, after which the dog will roll over and play dead.
People also have idiosyncratic commands that they teach or develop for their pets in response to situations that occur in daily life. I've taught my dogs that 'excuse me' means move out of my way. I was thinking about visitors to my house who might not be used to dogs, and what they would say if the dog was in their way. So I trained 'excuse me'. It's one of my favourite commands. I like it because it makes my dogs seem so polite, and it gets used several times a day because they do have a habit of laying down right in the middle of the room or doorway, where it's awkward to step over them.
There's also the question of which are the dogs' favourite commands. It's definitely not 'excuse me', because that involves getting up and moving when they are settled down nicely in the doorway. For one of my dogs, it's 'pull', because he loves to play tug of war. When we say 'pull', he pulls even harder and puts more effort into winning the game. He loves it. As for our other dog, he looks very proud when asked to 'shake a paw'. I don't think that's his favourite command though. I think his favourite is whatever command I happen to say when I have a treat in my hand. He loves his treats!
How about you? What's your favourite command? And what is your dog's favourite?