Gorgon, Goat and Bandit are the easies normal monsters, and dungeons which contain them are ussually significantly easier overall in my expirience.
Zombies are by far the hardest, because of the stats and being undead. Seriously, what IS the point of balance for zombies, there seems to be none -.- Everything else can be traced to a +/- HP, +/- DMG, +/- ABILITY, but they have both good HP, good DMG and since paladin does nothing against the undead, the only time it's any sort of downside is against a priests melee. Wut?
This is one of the things that should be looked into IMO, since it was known even back in alpha - Frank the Zombie was, by some margin, the hardest boss to deal with. And he made the Crypt dungeon harder than the Factory (which was ment to be harder). And he was nothing but a souped up zombie, with DMG/HP ratio simmilar to the SMM, but with more burst (and not hitting to hard was SMM's point of weakness, as well as the fact that you could poison SMM, while you couldn't poison the fat and robust Frank). In the least, we need some anti-undead gear, either a potion, a keg of dmg potions, a small item or a regular item.
Hardest non-basic monsters are ofc. Ice Troll and Animated Armor. Ice Trolls required me to pack the Really Big Sword to pass the witch quest pt. 5, and after about lvl 5 take too much resourcess to kill. And Animated armours...
Are another thing that needs to be looked at... In alpha they were a gimmick monster made specifficaly to annoy the living crap out of melee classes in the nominally hardest dungeon (event though the cryt was harder due to having Frank around). They were meant to be avoided by them, and used as insta catapult fodder for casters. But if I remember correctly, they alway's had 1 HP (except in the Boss Hive, the only place besides the Factory that had them, and there every monster had "boss stats"). They were the Cydstep-Rogue of monsters, excet now they're Cydstep-Dwarven-Rogues which you can't regen fireball, and still can't melle past a certain (low) point. And they're all over the place, which is why i play as much assasin as I do - not having to pay attention to them, and still get some XP is a huge leg up in any level where they are common.