By fastest I mean fastest in terms of experience gained/amount of real time spent. EDIT: I have completed the main story and am currently in the post-game. I have also unlocked the option to change the team leader and have access to most legendary Pokemon.
If you just want to level up and don't mind using joy seeds, this is probably the fastest way: https://www.youtube.com/watch?v=jImNyd4X-WE
(you should recruit raikou first/bring an escape orb to make it out safely)
Edit: Takes about 10-15 mins for each run which gets you 16 joy seeds. Although I haven't tried, in the video he says you can get enough for level 100 in an hour.