"Hey!"
"ArghgetbackherenowhywouldyouARGH!"
"Don't even think about... NO!"
I am guilty of all of those. Seriously though, I don't have one go to command. I have a bunch, and they vary on the situation and which dog is in question. "Hey, come check this out," invariably brings them all over to investigate. If they are already close by, "sit." If it's Chewy and I want her attention, then it's, "sit, give me your paw, other paw," back and forth for a little bit.