Well, I never did math on that, so following is based on test experience and hypothesis.
Ettin and Ettin archer are more or less the same, archer's shots are weak and won't cause much danger, unless there's huge number of archers around coming from different sides.
Ettin flechetter may quite be quite dangerous when in major number and standing on elevations or behind ally backs. In close combat they cause some irritation for placing time bombs, but careful player may easily avoid that; in both cases flechetters may kill lots of allies, unfortunately <!-- s;) --><img src="{SMILIES_PATH}/orc9.gif" alt=";)" title="Wink" /><!-- s;) -->
Ettin Commander is a big step up the difficulty ladder, it kills 1st level player with single shot (hammer toss) and deals either 3x or 4x melee damage (don't remember now).
Afrits... well there are 2 types, I consider Afrit Leader obsolete, because its concept was not well enough for a separate monster type.
Basic afrits - you know them well enough, Afrit Martyr is totally different, a deadly kamikadze; both need open spaces for effective fight, although Martyrs maybe placed in narrow passages and niches behind a corner, since they act like a bomb.
Chaos Serpents (demons) are all the same, except Browns now poison player with their toxic shots. But since the game is overall tougher, I think it is ok to use them as before.
Stalkers are all the same, Magma Stalker is supposed to be put in lava, but it is not tougher than ordinary ones at the moment.
Centaurs, while tougher than in original Hexen (since they now can jump to player), have 2 unique additions: Flame and Spectral types. Flame one is more dangerous in close quarters (floor attack is very damaging, and actually I was considering weakening it), but is not so on long distance. Spectral can better prove himself on middle and long distances, because of his half-transparency and ability to slide to sides as bishops do. I can't say which is more dangerous in general though.
Bishops: Cardinals are improved version of Dark Bishops, but have similar behavior in general. But Advisors (Black Bishops) are totally different! - they should be considered half-bosses, like Heresiarch, and used likewise. Well, maybe they are little weaker, than Heresiarch, since their protective spell does not defend from all weapons. So, there may be more of them. Also, they will act much better in open spaces.
Hmm, what else. Other original monsters are pretyy much the same.
There are Ghosts that fly through walls and cast "confusion" spell on player. They are pretty weak, but may cause.... well, much confusion <!-- s:) --><img src="{SMILIES_PATH}/orc2.gif" alt=":)" title="Smile" /><!-- s:) --> They are better used in large groups.
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So, in conclusion...
Level 1
Ettin
Afrit
Level 2
Ettin Archer
Centaur
Green Chaos Serpent
Level 3
Brown Chaos Serpent
Stalker (only because of his hiding ability)
Stalker Leader (not really much different)
Magma Stalker
Ghost
Reiver
Dark Bishop
Level 4
Ettin Fletchetter
Afrit Martyr
Slaughtaur
Level 5
Ettin Commander
Flame Centaur
Spectral Centaur
Dark Cardinal
Whyvern (?)
Level 6
Dark Advisor
Heresiarch (?)
Level 7
Korax