The duty roulette low level it's the best choice for quick exp, not for the exp you get inside the dungeon, but for the massive bonus exp at the end of it.

I don't exactly remember how many xp, based on your current level, but I remember doing it with my WHM at around 40-ish, and at the endo of the dungeon, whatever dungeon it was, I always gotlike 100k exp point.

same thing goes for the DR of the guildhests, even if the amount of exp it's less, but it's still a fair amount of it (around 30k if I remember correctly)

apart from those 2, Hunting Log, 1 or 2 run of the highest dungeon you can do, in a day, and you get to LV 50 really fast.
even fates can be a good choice, with all the people farmin atmas everywhere, you get exp, and you get it fast because fates usually last 1-2 minutes, lol