Choose one best decision and one biggest mistake!
Best decision: Scrap 1.0 and rebuild the game (easy one I know, but I don't play much so I can't think of anything else). 1.0 was pretty dang bad haha.
Biggest mistake: Choosing the Trinity only over Trinity+Hybrid roles.
Doing this pinned them into a corner on how to properly implement their most popular hybrid roles. They now have to force jobs into a role, like they did with RDM, or do these limited jobs which I think the majority of us despise because we can't main the jobs we love. (I prefer just choosing a trinity role over the whole limited thing, but the ideal thing would be to add hybrid roles to the job system, but I think they'd never do that kind of big change this late in XIV)