Honestly i think it depends on your class and how much you like the world if it is easy or not. I have a storm that just started avalon, and a death that is almost done with celestia, since i havent done all the worlds yet ill just rate what i have done. Wizard City: Do i need to say anything really? Krokotopia: On storm it was a little difficult cause i couldn't solo a lot of bosses and things because of accuracy and health, on death it was very easy, i soloed (dont know if i spelled that right lol) almost every thing. Pretty much easy, just long. Marleybone: Hard for storm just cause of a few boss fights, and constantly getting pulled. Death was easy plain and simple soloed almost everything besides the counter weights katzenstein and big ben. Long and very boring. Mooshu: My favorite world, very easy goes by fast, had help on storm and death. Death soloed a bunch of bosses. storm did some alone. Dragonspyre(old). Insanely hard for my storm needed help all the time (didnt have a death character yet) Dragonspyre. Death pretty much soloed everything besides the dungeons, some what long. Celestia: Difficult on both characters, didnt really solo anything. Zafaria: was very difficult for storm because of my health and their health. Overall it seemed the worlds i didnt like were the hardest, but were somewhat easier on death, i quit for 2 years so i dont know if the updates made it easier or if it was the class but death seemed easier for most of the worlds.
Well I'm on level 36 so my hardest world is marley bone and in there katzensteins lab is really hard. I have been to mooshu and dragonspyre and those places are HARD
On my first wizard ( death ) celestial was tough I was just 50 and starting that new world. without a way to block critical hits at the time this made is a real problem. and trash battles started with 3-4 pips (this was before ww existed and the level 56 crafted armor)
In today's worlds i would say the last 20% of az because now i cant use my starter deck to take out basic fights anymore :-) sadly I had finished az before getting the new may cast fairy talent into my pets lineup. Having super pets can make the difference in your wizard experiences.
My last new wizards roll out with proof, defy, sprite, fairy at least on their pets and another wizard to speed up the process some ;-)
It really depends on how you progress through the game. I.e., how many side quests you do (including side worlds). Celesita is probably the best example of this. If you go directly to Celestia from Dragonspyre you will find yourself in for a lot of... pain. I did that on my first wizard (ice) and found Celestia to be extremely difficult at times. On the other hand, with my third wizard (fire), I'd completed both side world arcs (Wysteria and Grizzlehelm/Wintertusk), had the Wintertusk crafted gear equiped before I started Celestia. Doing that, I found Celestia to be one the easiest worlds instead of the hardest.
The order that I would recommend most players take is:
1) Wizard City - Unicorn Way through Colossus Boulevard 2) Kroktopia - Pyramid of Fire 3) Wizard City - Crab Alley and Sunken City 4) Kroktopia - Krotospyinx 5) Wysteria 6) Kroktopia - Tomb of Storms 7) Marleybone - Digmore Station 8*) Grizzlehelm - Savarstaad Pass, Virgid Roughland and Mirkholm Keep 9) Marleybone - Scotland Yard Roof and Big Ben 10) Mooshu 11) Dragonspyre 12**) Grizzlehelm - Ravenscar and Wintertusk 13) Celestia 14) Zafaria 15) Avalon 16) Azteca
Completing worlds in that order you will find that the difficulty is "appropriate" for you level at worst, easy at best. (other than Azetca, and to a lesser extent Avalon, but that's a separate issue)
* Completing Digimore Station is only the best point imho to start the Grizzlehelm arc. You might not be high enough level to complete all three of the first areas at once. Its close though, I remember getting my level 35 spell quest right as I started Mirkholm Keep. In addition you could use them to break up Mooshu a bit if you wanted
** I only put Ravenscar here because of the first (Jotun) and last (the Coven) fights, if you do Ravenscar after Dragonspyre you will find most (if not all) of Ravenscar other than these two fights very easy.