Well, I wasn't around for those older threads so I don't mind a new discussion.

I have found mages and commanders to be the most useful, especially later in the game.  Assassins, Defenders and Warriors can be useful early in the game, but by later in the game, they are mostly just standing around with their hands in their pockets.  Except for dragon battles, where they are less likely to be struck dumb with fear.  They can be very useful afainst opponents with fear, but in all other battles late game, I find trained troops to be so much more powerful than Assassins, Defenders and Warriors, that I rely almost exclusively on trianed troops, and let my champions take a break.

The issue I see with Assassins, Defenders and Warriors late in the game is that, even when fully tricked out with the best gear and traits, they are not nearly as effective in a battle as a troup of common footsoldiers armed with the 25 damage maces.  You can train a troup of six units with leather armor and the 25 damage maces in just a couple of turns, and they will have around 100 or more health and do 150 or more damage (depending on what improvemetns and enchantments you have on your fortress).  Late game Defenders can withstand a little more damage, but they cannot dish out much damage (unless you happen to be lucky enough to have found one of the rare superweapons, like the one that lets you cast blizzard).  Assassins and Warriors can dish out more damage, but don't have the health to stand up to the enemies militia.

I've got a game going right now with Gilden and Lord Marken has something like 125 health and a lot of defense, but he can only dish out 25 points of damage, whereas his foot soldiers have over 200 health and dish out nearly 200 points of damage.  HE is on a horse, so he can charge in and get in a few quick swings, and he can stand up to a lot of  damage, but he can't really kill anything tough.  Most of the time I don't even bother moving him because he just gets in the way of his footsoldiers and prevents them from being able to attack an enemy.  So, I just pass through his turn and leave him in the back where he "direct" his troops but he does not really "do" anything.

When I have played assassins and warriors, they would also hide in the back most of the time for big battles, and maybe swoop in at the end to take out some troops once the ranks are thinned.  The issue with assassins and warriors late game is they cannot take a hit from the tougher troops your enemies train, that can do over 100 poitns of damage in a single hit. 

I like to get at least one champion who can cast guardian wind to protect my troops from enemy archers, but that is mroe of a mage skill.  Beyond that, I use my trained troops almost exclusively, unless I am playing a mage.  Commanders are great because they are supposed to stand in the back and command and their traits let them provide a huge benefit to the army jsut by being there.

Ahh, you've very welcome.  I am surprised you have not gotten more response.  It's not like you asked a question that has a definitive answer, like "how do I increase the power of my fireball spells"  You asked for a discussion of people's opinions on a question with no inherently "Correct" answer.  At the Bethesda forum where I usually hang out, you'd get get a bunch of answers to a question like this, even in the forum where they only discuss ancient games like Arena and Daggerfall and even though the same question may have been discussed a hundred times before.  People that love the game just like talking about this type of stuff.

I just started posting here a few days ago and it feels like I have walked into a church on a Monday morning and no one is here -- it's sooooo queit. 

I agree with what Steve M wrote.  No matter what faction I play, I start my sov as a Mage.  When some heroes come along, I choose one as a side kick.  This one chooses skills that increase the effectiveness of the army.  This becomes core force.  Magic is just too powerful not to maximize. 

Henchmen are very useful, as they're essentially design-your-own heroes. You can give them more or less any of the traits that regular units can be trained with, they can use items like standard champions can, they gain the bonuses granted by the city they're trained in, and you can give them a much better version of Potential at training time. They can learn real magic if you have access to New Pariden's spellbooks (either a custom Race of Men faction with the Decalon trait, or access to a a faction with that trait that researched the required techs for the spellbooks to appear in the shops), and have a decent set of minor magics. Train them in a decent fortress and you have an essentially limitless supply of level 5-ish Commanders to put in your towns or buff your armies, and unlike stacking normal champions with one another, stacking henchmen cause no additional reduction in experience gains beyond the effect that army strength has. They're also in an easy spot of the tech tree to reach, they're immortal, and equipping them with the best weapons and shield your faction has to offer doesn't cost gold. (Also, the training them in fortresses thing isn't just with Commanders; it'll work with all classes. Commanders are just the ones which give you the most benefit for doing it; Warriors, Assassins, and Defenders are kind of niche, especially if you already have decent troops, and generally speaking true Champions are better Mages early on, since Mage Champions typically come with a spellbook or two, which means less gold blown in the shop, although the Magic Adept paths that Henchmen can have provide them with adequate access to spells, and there's an advantage to being able to choose every trait that a Mage gets. Henchmen additionally have some decent army-boosting traits that nothing else gets.)

Sions, on the other hand ... I can't say that I'm terribly fond of Sions. They're much higher up in the tech tree than Henchmen are, they are (or at least were, the last time I bothered with them) more expensive than Henchmen are, they have no real access to magic, and they're much closer to being regular units than to being champions (for starters, they die and you cannot equip them with the stuff you find in loot). On the other hand, the description of "best Fallen warriors" is theoretically true - on a per-figure basis, if you train them out of a Fortress with the unit buff enchantments, they'll have the highest base attack and defense skill of any of your units, after they get a few levels on them, provided that your champions haven't been reading too many attack and defense boosting books, but the real champions get access to much better weapons, which more than compensates, and the real troops are around in greater numbers and are easier to mass anyways, which also more than compensates since that both negates the per-figure damage advantage and greatly overshadows the Sion on the HP front.

With the mods that have new spellbooks, sometimes the new spellbooks can be bought with The Decalon, too.

I found that a good magical counter to an archer empire like Tarth is to equip a dagger (for the +3 initiative bonus) and whatever else I have to boost initiative and then firing off a quick Guardian Wind spell at the beginning of battle.  A mage with light armor, soldiers boots, a dagger and maybe one item like a backpack can usually get first action in any combat.  That early Guardian Wind spell can mean the difference between victory and defeat.  When playing Resoln, a quick Wither spell can have much the same effect, so if the archers are getting the jump on you, check to see what weapon you have equipped.  My mages used to equip ranged staves, but they don't anymore because the initiative penalty those carry is just too restrictive.  My mages almost always wield daggers now, and there are very few enemies that can get the jump on them when battle starts.