Not sure there's a consensus and there's a big bunch of factors to take into consideration.
EDIT: I ranked stuff from easiest to hardest
Assuming you have everything unlocked, I'd rank the viciouses as:
Although I'm not sure, because I've never done full completion on all of them and VGT is more like a roguelike of it's own. There's days when I'd swear Daemonic Library's the hardest, and I had really annoying Dragon Isles runs as well. Vicious Steel would also probably be a right pain it wasn't for the damage reduction gimmick.
To be prefectly honest, I'd rank Daemonic Library, Dragon Isles and Namtar's Lair as harder than VGT, personaly. I find doing them takes me more mental effort and starting a run in them makes me feel more dread.
As for non vicious dungeons, hmmm... Never really though about this, but:
1) Hobbler's - Because doooh
2) Southern Swamp - Because fixed enemies, no gimmicks whatsoever, one really easy boss and harmless plants.
3) Northern Desert - Has no impactful features except it could screw you over by spawning a blob, now has 2 boses.
4) Venture Cave - Easy and relatively spacious.
5) Western Jungle - Would be rather nasty if it wasn't so close to the start. Harmless plants are harmless though.
6) Ick Swamp - The only threat in this place besides vampires is your own psyche. Fun, time consuming, easy.
7) Eastern Tundra - Has a boss that can hurt you, life drain enemies and a bit of a nasty layout
8) Berserker Camp - Honestly, I couldn't tell you what it's gimmick IS (berserkers level up somehow and there's a bridge?) I have more problems with the Den of Danger, really.
9) Rock Garden - Also strangely easy, but at least I know what's it about (3 bossess and a labyrinth)
10) Den of Danger - Errr, it's cramped and has a wide selection of bosses. Tricky VT run, up there with the really nasty ones for me.
11) Doubledoom - Random bossess, iffy layout.
12) Halls of Steel - One really easy boss and a fixed monster roster. Cramped though.
13) Magma Mines - Fixed gimmick bossess, but no subdungeon + screwup potential
14) Labyrinth* - If prepped for it, pretty easy, if not - it flat out tops the list for me (+ no subdungeon?)
15) Hex Ruins* - if you have a solution for the revenants - decent, if not - pretty challenging + no subdungeon.
17) Havendale Bridge - Rather standout in difficulty and required thought for it's map position. Enjoyble.
18) Grimm's Grotto - cramped but I always manage to beat it somehow
19) Slime Pits - as Darvin put it "a cesspool of fatty monsters and nasty debuffs". It was made to give the monk a hard time, and monk is a godmode cheat sort of guy. I don't usualy fail SP, but they're always tricky
20) Cursed Oasis - is a nasty place with a difficult to ignore gimmick which makes you play it's game
21) Shifting Passages - I can't remember the last time I failed it, but as someone put it "shifting passages can go die in a fire". One of the few dungeons where you can end up hoplessly blocked off, and so on and so forth
22) Creeplight monastery - I honestly find it the most difficult dungeon, one boss with 3 forms who gives no xp (if I remember correctly), and low XP overall. So many things that let you beat other places just don't work here. VT run of this place was tiresome to say the least.
This was fun
Ick Swamp has been giving me tons of trouble so I'm glad to see it in the top 10 hardest dungeons on your list
You're not the first erson I hear this from. What seems to be the problem? The only reason it's not right above Hobbler's for me is that you actually have to know a few basics, and that Southern Swamp and Northern Desert are no-extra-features strictly easier variants of venture cave (because of fixed monsters). Otherwise it has always been the way easiest dungeon in the game for me.
There was a situation where I had problems with it, and that was when vampires used to do magic damage. Othewise, I saw it as a remarkably easy dungeon even when the boss was tougher (was it?) and when they swapped the places of Ick and Hex on the map I couldn't believe it because I couldn't figure out how can that concept turn out more difficult than the one in Hex, even in theory. It's also really obvious when you try to do a VT run of Hex (darned tricky) and of Ick (meh). Can anoyne clue me into what's confusing everybody?