Let's not forget immunity to stuns of any kind
Didn't that require a 'sploit where you had to get hammered by Black Deaths until you had so little max HP that the game assumed you'd be OHK by practically anything that hit you, but failed to check if you had MS going, so it didn't put you into stun for what it (incorrectly) assumed to be the death blow?
Never, ever, stand between a mage and his enemies.
You learned that the hard way?
Is there any other way to learn that?
So what are the relative power levels between the classes?
Sorcerors rule, I've gathered, but where would you place the Fighter, Rogue and Monk on the list?
At "fun to play".
Rogues are generally seen as a n00b class, though. Very fast with bows.
Fighters are melee tanks, they can easily stunlock most things, and have even better staying power than their HPs and AC would indicate when using shield. Magic mostly for support (like killing a cluster of succubi), although if you get a lot of good +magic items you can learn teleport and melee everything you can see.
Monks are quite well rounded but suffer from diminishing AC returns from heavier armors. Very good and rapid unarmed attacks, great with staves too, can block and attack 3 adjacent tiles in front when wielding staves.
Search as class skill.
There are also two hidden unfinished classes in Hellfire - Bard and Barbarian:
Bard is jack of all trades and can dual wield for massive damage, but can't swing nearly as fast as fighter so melee can be more perilous than with Fighter (stunlock or be stunlocked), so you'll need to alternate between different approaches. Also ID as class skill. Can attack 3 adjacent tiles in front when DW.
Barbarian maxes out at 0 magic, has no mana (you can only cast anything with barb if you have +magic items equipped) and is sort of fighter on steroids, can wield 2h swords and mauls in 1h, murders shit with axes, and, if not wielding a shield, attacks 3 adjacent tiles in front of him. Has some half implemented berserk mechanics as class skill, but also gains resistances with levels, has tons of HP and massive max vitality.