For Civ 6, I’d say winning each victory once. Try to do it with different civs each time too. You can set your goal as winning a game on the highest difficulty if you want, but personally I don’t find that to be as interesting as the shift in gameplay necessary to win the different victories without just militarily crushing everyone else.
And they intentionally shake the gameplay up with each installment. I’m glad they do. Otherwise there’s little point in doing a new one