Most defiantly Celestial. Ok, here's my story... I was going to fight Tangleroot on a death guy, right? Well, I get on the sigil, and I'm frozen. I can't move or anything. So I'm like, What the... And then I made a new wizard, got him up to Dragonspyre, and thought, Hey... Maybe I can just... And I equipped my mount, and I was fine. Well, days later, I lost my membership, and didn't get it back until 6 months later, so Celestia took me about... 7-8 months
mooshu because travel times were so freakin long even with mounts (i can even imagine how a wizard without a mount could do it) and so much running back and forth.
celestia because of the sheer shock of it at the beginning... the difference between DS and celestia... its where mobs starting having so much more helath and 6+pips with 7 pip spells.
azteca... just insane how long a battle takes. just not fun.
marleybone because EVERYONE has weakness and shields... and you just keep getting pulled into battles on those rooftops.
avalon because the bosses became outrageous. more cheaters.
zafaria because you did go back a hundred times to fight the same thing. (this actually rfeally started to be really bad in celestia and got extremely worse in zafaria)
grizzleheim because there was no experience. (and that first storm boar mob was outrageous for level twenty... 600 health while the others were at 300, they had more than the first boss if i remember right)
i actually enjoyed krok, dragonspyre and wysteria except for the second tree man mission
my favorite world, by a long shot, was wintertusk.
Well there was that one year on my Wizard that I dragged out dragonsypre. I wouldn't do any towers by myself for some reason, idk . Eh I was younger but anyway right now the world that dragged out for me the most was .....
CELESTIA!!! Goodness gracias celestia was so difficult for me. But I managed to face my enemies without dying. It took me almost a YEAR!!!!!!!!!!!
For me marleybone was a long and boring world and the mob fights were impossible to get past them, they need to make a side walk or something, plus i was storm so i fizzled lots of the time.
I'm not sure about you guys, but I zoomed through all the worlds up to Avalon without any trouble. (Solo or not.)
Avalon was okay, but then, there was Azteca. I've been in Azteca for about as long as its been out and boy oh boy, I'm looking forward to the day in which I can leave it behind. It's hard to solo and it drags on and on and on.
Wizard that I dragged out dragonsypre. I wouldn't do any towers by myself for some reason, idk . 
Eh I was younger but anyway right now the world that dragged out for me the most was .....
It took me almost a YEAR!!!!!!!!!!!
Jasmine Roseflame lvl56 
Savior of The Spiral 