Apparently it's all the rage to post on Descending AC versus Ascending AC. I am certainly not above the debate and I've certainly gotten embroiled in it in the past. How AC is defined and used in the game should be part of that debate.
What do you want from your armor class score, your AC? Do you want a handy dandy score that makes to hit rolls crystal clear? Do you want a relative index that makes things easy to gauge?
AC as Index:
If your game has a combat system that uses charts it simply doesn't matter. The AC score is an index it could run from A to Z, I to IX, 9 to 0, 1 to 100, 10+ and it wouldn't make a difference,
combat resolution would be based on consulting the chart. An index can be used a Fixed index or a Floating Index.
When using AC as a Fixed Index don't use it as a floating value. This is a place AD&D screwed up with the weapons vs armor chart, it added a layer of complexity that didn't help game play. If you really must use it as a floating value keep all your bonuses going the same direction (If you have a +1 shield being a boost then a stat modifier of -1 boosting AC does add confusion).
An (old) Trick you can use with Fixed Index AC scores:
Attacks vs armor charts- If AC m always means someone is wearing chain mail and AC p means banded mail and large shield one can set up a combat chart that lists different attacks with to hit scores versus different AC scores. If the AC value changes the utility of such a chart breaks down.
How to Use AC as a Fixed Index and have modifiers for size, agility and such:
In such a system the AC score stays fixed:(AC 5 is chain mail. AC 3 plate) but one also records the total of modifiers along with it. These modifiers alter the target number or the to hit roll depending on the range of scores your game uses. AC 5+3 meaning a target in Chainmail that requires a score 3 points higher then listed works with little effort and immediately sorts out issues that occur again and again with AC systems (What's your AC with no DEX bonus?).
Ascending AC scores:
If your game uses ac as a target number and keeps track of folks attack score and resolves combat by dice roll+attack value >= AC means a hit then ascending scores work better.
Ascending ACs do make it difficult to visualize how different AC scores are. AC 18 doesn't seem all that different from AC 21 but AC 2 vs AC -1 definitely are different. Ascending AC does have the advantage that it is indefinitely scalable, the numbers can keep on going up; however eventually the numbers don't mean a thing other then "your adventurer must be this level or higher to play".
Ascending AC and Dividing the AC:
I've seldom seen this done and it does alter play significantly. Divide AC by the number of opponents engaged when in melee combat. So if one has an AC of 18 vs a single foe (and this is the standard score) they would have an AC of 9 vs 2 opponents. This method get rid of the need to keep track of bonuses for multiple foes and makes ganging up on opponents a very sound tactic. One could limit the max number of foes that could attack or put a floor on the score.
example: Thogrok the mighty is AC 40 and is set up on by 10 foes this would mean an AC of 4 vs these foes in straight division, luckily for him he is playing in a campaign where ACs can't be divided to less then 10. This method lets a large number of opponents have a good shot at a high AC defender, widening the level spread that can participate in adventures. Don't use it for ranged combat, you'll never get PCs within 100' of opponents if you do.
Sacrificing AC for additional chance to hit:
This option comes up a lot and it works well enough. Subtract 1 or more points from AC to add one or more points to the attack roll.
example:Thogrok the mighty is AC 40, he's fighting soemone else who has the same AC that may not fight as well so he sacrifices 10 points of AC to be +5 to hit.
this method can work with other AC systems but the math isn't as quick or clean.
Descending AC scores:
Starting high and going low is an oldie and a goodie when it comes to recording ones AC. It does however introduce a range of issues. Some folks can't grok lower in score is better, they just will never get it. I myself see no difficulty with a lower number equals a small chance of being hit but not everyone sees math the same way.
A common problem with the Descending AC system is all bonuses being noted as a "+", think of it as a shift along the to hit chart and it's a little easier for folks to visualize.
Descending Ac scores are often used as a floating Index and sometimes as a Fixed Index (see above for notes on that). Keep bonuses going the same direction +'s mean good and -'s mean bad and things are fairly smooth.
Descending ACs do give one a clear cut-off point for typical and special, the move from positive to negative numbers is very obvious and at a glance is easy to visualize as special. The scoers if 18 vs 25 just do not look as different from each other as 3 vs -10, it could be nostolgia talking here but it is also cleraly evident.
THACO and the Descending AC:
It originated (along with other variants like THAC9) to make combat quick and not require a chart all the time. It works but not well. Folks often miss the simplicity of THACO style combat: roll your to hit if you'd have hit the # then you clearly hit all ACs that don't offer greater protection (If THAC0 is 14 and you get a hit roll of 15 you have clearly hit every AC worse then AC 0) no math is required. When you number is over thaco or under you have to do math some math.
THACO combined with a Fixed Index weapon vs armor chart doesn't work well because everyone always has to know what AC they are attacking and the convenience of THAC0 is lost.
Another issue with THAC0 is it poorly reflects tables with special results at extreme ranges of the action. The repeating 20's of AD&D being one example and the damage bonus for certainly going to hit in BECM d&D.
Here are some nice little house rule tricks you can do with descending AC:
Negative AC scores reduce damage. So if a combatant is ac -2 and is struck for 6 points they only actually take 4 pts of damage. One can rule that at least 1 point is always inflicted or magic weapons always do at least their + in damage as other methods where negative AC grants damage reduction.
Magical Weapon Threshold:
Negative ACs require magical weapons. Folks can't hit or do more then minimal damage (1/2 roll or simply 1 point) unless they are carrying a magical weapon of the inverse rating AC or better. So a +2 sword is needed to hit AC -2, a +3 sword is needed to hit Ac -3 and so on, hits made with non-magical weapons do but 1/2 the die roll with no additional bonuses allowed.
This works best when negative ACs don't stray too far from zero.
Yes it is completely possible to do the above with ascending AC systems but you have to do more math (oh no) or have separate scores for things like "damage reduction" or "magical threshold" . It's nice and handy to have a single score keeping track of such things.
The truth is there is no better method to AC. It doesn't matter if it ascends or descends it is just necessary for the GM and players to understand what AC means in the rules they are using.