Dungeon XP has been increased across the board, and the new duty roulette gives a very large XP bonus once a day for a random low-level (everything before Castrum and Praetorium) dungeon, and a random guildhest. The daily low-level roulette will give you a full level every time at lower levels, and a big chunk of a level at higher levels. That chunk will increase when 2.15 comes out in late January, because right now, the roulette favors the low end (Sastasha through Halatali), which means less XP within the dungeon itself; 2.15 is changing it to use more of the dungeon list. XP from battlecraft leves has also been increased substantially. Oh, there are also the beastman daily quests you can do starting at 42. I've only done those on level 50 jobs, so I haven't really paid attention to the XP, but it's likely comparable to regular sidequests.

There's nothing that's as fast as FATE grinding was before 2.1. Even FATE grinding. People aren't FATE grinding nearly as much now, which means that goes more slowly. If you do happen across a large number of people wanting to grind FATEs, that'd still be the fastest way to level, but you're probably not going to find that every time you log in.