Gaan'Telet's it's own category. If you manage it on your own, do post about it. It used to be rather doable before they changed it completely, now I have no clue what to do with it (except scumming for a GG Rogue and then something something). Used to be my playground
Now it's DD meets Kaizo Mario.
I'd say Naga City's the easiest as it falls more into puzzle than dungeon territory. This was my opinion before they apparently nerfed it, so it's possibly even easier.
I'm not sure about the others as they've changed since the last time I played them.
Dragon Isles used to be easy for me, except only a few classes could really do them. It was more stupid than challenging, the way it used to work, and maybe still does, with all the fat monsters in the game in one place and very little exploration.
Namtar's lair gets the rep for being rather easy on the forums, but I hated it. Playstyle and strat prefference thing I guess.
I'm not sure what' the current DL is like, it used to be the easiest, then it was insane, then it got nerfed a bit. Avatar used to make a mockery of it, but then again he's Avatar.
VHOS - was supremely difficult for a while for some reason I can't remember right now. I know I didn't even unlock the vampire on my last playthrough. Not sure where it stands now, some people say it's easy. Not sure why.