don't listen to derek and his propaganda. i've heard he is a liar and he sucks; i get teles on w1 warning everybody that he likes to plant spies in the other alliances, so he doesn't even have to sweat for a win. that he duels old ladies for pocket change, and once shot a blind man's dog because he fancied a snack. ok, the teles part trashing him was true; he's also a flake, a good player and a friend
back to your question, depends on what you want to do and how you want to play. w1 and w15 are worlds for more purist players (no skills buying, as the scoundrel pointed out). w11, w12, w13, w14 have been a paradise for nuggeters. that means a whole lot more skills than levels. w11 and w12 were known as fort fighting worlds, but are currently going through a rough patch; they are still a decent place for crafters and questers because they have enough active players to trade and get help. w13 is still a good fort fighting world, tho i think someone has plans to screw it as well. w14 is nuts, active in terms of dueling and fort fighting, but it changes with every moon. w15 has some decent battles, and is pretty good for duelers who want to score high damage, because it's full of tanks and high level crafters (that means, access to polishing stones), but market prices tend to be insane. w16 is very active, in terms of players, but battles suck there or so i've heard.
hope that info helps you with whatever your decision is.