Of all the revealed questlines, Pally's seems easiest to complete so far. Only restriction of playing 1 drops seems simple enough, and since I'm in wild, it's made infinitely easier to have more draw/refill options available. I don't see palladin's questline being difficult to complete at all.
The only real question is will the payoff for said questline be viable/worth it to win consistantly? Dude-adin builds can be strong but in hyper aggro formats it might struggle to make their big payoff turns before they die.
Mage questline should be ok in wild because Sorc Apprentice will always be a card there. Until she finally gets the nerf bat swung at her she'll be a huge nuisance but make this deck stronger anyways.
All the questlines that are revealed so far are fairly easy to complete with exception maybe to Mage's Sorcerer's Gambit because you might not want to always cast or even put that many of one type of spell. Cast one many type of one spell and you will be locked out from completing the quest.
Priest might take the longest to complete but that is your gameplan with it anyways. Control the game until you cast a 10 mana win the game.