Kill/collection quests are the quickest. You can use runestones to increase the experience you receive from enemies, but they're pretty expensive (especially the 200% one, at about $5 for five hours). I've played periodically for about a week and hit 32 earlier, so it's not a huge time sink as far as experience earning goes, but recently there have been fewer quests and I've needed to grind more. No idea if that will continue.
Make sure, especially if you're a ranger or mage, to hit 10/20/30/etc. before moving onto each area. I entered the fourth area at level 28, and it took a while to adjust.