Kingdom Hearts, first game, go in release order.
It’s not confusing at all this way like everyone who’s only played 2 and 3 says so.
Only complicated thing is that there’s games in between the numbered entries and they’re crucial to plot and are not side games so it’s stupid they numbered only some of them. Even 2 wasn’t the second game so they didn’t need to call that one 2.
For final fantasy, quite literally just pick whichever one has a theme and/or gameplay style that looks interesting to you. As long as it’s not a sequel to a previous game you can pick any.
The only ones with sequels (some prequels) are VII, X, XII and XIII.
Although some could argue with XIII you can also start anywhere.