Me and my FC did some testing, running the dungeon closest to your level gives the most xp in one go, can be even more than lvl roulette depending on which dungeon you roll in there. Best xp vs time would be fate grinding, but only with a group, on your own you take a lot longer to finish. Potd is a nice alternative too, especially on lower levels. We didn't try pvp (not really our thing) but I hear there's a lot of xp to be had, if you like this kind of content.
Personally, I do my roulette and then a mix of all these options. If I stick to just one thing, I get bored fast.