Heavily indulging in bounties and your stronghold questline will easily be the fastest route to XP, since both can be done rather fast compared with completing a multitude of seperate quests/sidequest lines, all the while buffing out your beastiary and possible hoard of traps/locks you find on your way. That is alot of XP! Also helps in my case the bounties to be great fun [especially the backdrop to them], something they should definitely flesh out more.
However if you want to build an Arcane Warrior (Proper AW, no cypher junk) you want the XP asap on a wizard! Takes lvl5/6 spells to fully flesh out the ideal character. That needs fixing