Hell Ancients, in my view - Normal Diablo being a very close second.
The reason is simple - the Ancients have 2 random mods each (and currently, if one is FE, is instant death to any character), spawn with rare weapons (ouch), and there are 3 of them, making it hard for a character (other than a Summoning Necro/Druid) to keep them split up.
Normal Diablo is close, because of his high HP, blocking, and dangerous attacks. On /players 8, however, he's not that tricky (dangerous yes, but with enough patience and town trips...), because you'll be past level 30 already. On /players 1, it may well be a different story.
Since I've not reached Normal Baal, let alone Hell Ancients, yet.. I've voted for Normal Diablo. Certainly my Fishymancer found him harder to beat than any character I've ever played did.. which is scary, since this character normally sweeps aside all enemies easily, even Act bosses.
An interesting thing is that most Summon-based characters (especially Necros, who need corpses) will be eaten by Normal Diablo, but will find the Hell Ancients easier due to outnumbering them, and not needing to melee (so no risk of the FE bug). So it depends partly on class & build.
normal diablo is harder for most of my characters than normal baal, and the same fore nm and hell. act 5 you gain alot of lvls and so better attck, and baal as long as you dotn let him spawn his twin to often, he just takes time.
Normal Baal for melee characters without an always hit skill, he'll just block the few times you manage to get a hit in. Hell Ancients hands down otherwise...well, except perhaps Baal for my ES sorc, since he uses Blood Mana.
Definately Hell Ancients. Take away the FE bug and they still are the hardest in the game. Why? Because it's you and them. You can't leave without reseting them, and the spawn of mods can be utter rediculuous.
Norm Lister and friends (not other waves):
This guy just smokes melee characters, especially the first time you fight him. ITD, stun and KB. M'Kay, have a nice life.
Not so much as D himself, but getting there is a long and trying process. Melee characters fear IM, and everything in there just hits hard, not to mention the huge damage boost the Balrog's Inferno got. IMO, that's the hardest of the three seals.
Right now he is a pushover, but I remember him before the 1.10 patch.
Back then shaman bosses spawned with a shaman boss pack and running into about 6 simultaneous infernos could tost moth characters and mercenaries.
Diablo is also so nasty in Normal because you usually do not have very well developed resists and hitpoints. Thus his undodgable fire nova can actually do damage. Skelliemancers are in an even worse situation because their only class-related damage source is stripped from them. They have to rely on a merc that dies ever so often (usually being toast by Diablo's pink lightning).
From the list given, and based on the only ones I've met, I give the nod to Norm Diablo. However, having gotten my head handed to me on a plate by NM Ancients I'm not looking forward to the Hell rematch.
I'm torn between Normal Diablo and the Normal Council. They can be quite dangerous, even luring them out one at a time. If they all manage to see you and put down a hydra, they can get you fast. Couple that with the poison spitting reptiles, heirophants, and Dark Lords and you have one tricky situation.
I'd say Hell Ancients; with good planning (build lots of FR/LR before taking him on), you should get past big D without much difficulty. Hell Ancients are extremely tough-even without the FE bug, melee characters are in for a headache against them.
And Normal Coldcrow is a beast! That cold attack just freezes you, and I have come close to dying there a few times already. The skelliemancer and EL druid had the easiest time, the Pally the hardest.