[qaccy]

I could make...a long list :O

From a solo, melee-oriented perspective:

Alby Beginner/Alby Normal (honestly, they're practically the same)
Ciar Beginner
Rabbie Normal
Ciar Basic
Alby Basic
Rundal Normal
Rabbie Basic
Fiodh Normal (Gorgons suck, but you usually see them only once per dungeon if at all)
Barri Basic
Ciar Intermediate*
Math Normal (Despite being stronger overall, poison kobolds don't multi-aggro unlike their goblin brethren)
Ciar Normal
Barri Normal (Imps...annoying)
Alby Intermediate*
Rundal Basic
Rundal Siren
Coill Normal (Try taking on three different sprite types as a melee character, I dare you <_<)
Math Advanced
Rundal Advanced (So...much...death)
[Imaginary dungeon to increase the difficulty scale]
[Imaginary dungeon to increase the difficulty scale]
[Imaginary dungeon to increase the difficulty scale]
[Imaginary dungeon to increase the difficulty scale]
[Imaginary dungeon to increase the difficulty scale]
Peaca Normal (Yes, it's that hard 8D)

*This is based on the average difficulty of all three Intermediate passes with an appropriate-sized group for each

I think that's all the dungeons, with the exception of Albey and any special-access dungeons (most of them are derived from an existing dungeon anyway). I also didn't list Fiodh Intermediate because I've personally never done it (hooray for not needing FB/AR pages!). Judging by what's in there though, I'd put it near the top of the list for difficulty.

Again, these are all based on MY PERSONAL OPINION as a melee-oriented character (with a little magic purely for pulling/countering purposes). I have no advice for other character builds.