Honestly, I would count Primals because for one thing. They're not easy. I doubt you'll pass them on the first try. Maybe Ifrit or if you have a team to carry you.

But if you just reached end game. The best weapon you received is the unfinished relic weapon and level 50 Grand Company weapon. So you might have to grind Ifrit/Garuda just to get a better weapon or do Wanderer's Palace which also provide Philosophy/Darklight weapon.

Another thing is the Relic Quest. You have to fight Chimeara and Hydra, then Ifrit/Garuda/Titan HM.

Plus you can also spam Castrum/Prae for Philosophy which I did for a while until I can do AK. This is just the beginning though, eventually SE will add more of course.