Leves are the quickest, but are you also a crafter? If so, gathering high yield stuff (i.e. darksteel ore) means you can CRAFT high yield stuff (darksteel nuggets) and then sell a more valuable product on the market place. You get experience for gathering AND crafting AND you make money. And sometimes you just want to do some brain dead gathering for 20-30 minutes. Get into a rhythm and whack away. And like Roth says, GC hand in's are a nice supplemental source of experience AND you get GC seals. BTW, there's really no short cuts for fishing, it's just a pain in the ass, LOTS of monotonous grinding.