John Adams would probably be the first president.
However, things would be a lot worse. Washington's decisions, like making a cabinet, or stepping down after two terms, would not be in place. Washington was also the hero of the nation, and had many fans, so we could see a worse Whiskey Rebellion with Adams as commander-in-chief. Such problems with his presidency lead to an earlier alien and sedition acts, and will make the role of president an unlikable position. I think things would be more unstable in the young republic.