it depends on your level and where do you play and also the class
I would go for Awesome plate with good basic armor. You can find some Awe.full plate mail from Gris for about 100 000 gold around level 30 (I think)
Obsidian plate (of stars pref) is used by high-level warriors for magic only levels AFAIK
Sorry, i should have said. This was on my lvl24 warrior. Trying to do hell runs at that level even on normal don't seem to warrant that price tag on a -4 to enemy plate? As i'll be fighting magic enemies a fair bit i like to have some resist all just happened to be on my old plate.
Awesome full plate seems to be a much better choice and cheaper than what seems a lot worse AC and -4 enemies.
I'll probably continue to do hell/normal runs to Laz with my warrior with the odd nightmare shopping/shrine spree throne in.
High Ac on a warrior isn't as important as your blocking. Sure, it helps, but it isn't absolutely necessary. I used to play hell/hell co-ops all the time using a Obsidian Stars leather armor Fun stuff.
Monster to hit is linear--that is, for every point in AC, you're 1% less likely to be hit (with certain limitations of course). This essentially means that if you forego an Awesome FPM (up to 187 AC), you can basically drop the idea of using AC at all. So Obsidian Rags of Stars would be just as good as Obsidian Plate of the Stars. This is basically for Hell/Hell, where monster To Hit is around 230-250. Other places are certainly easier, so high AC isn't a big deal.
If it's -damage you're looking for, try Scavenger's Carapace. It has -15 damage. Of course, when monsters are doing over 100 damage, this won't be as helpful as high AC or the mana you can garner from Obsidian armor choices.
Harmony can be helpful. It's better than not having Harmony, but not as good as Stars. Harmony reduces the time it takes you to recover from being hit from .25 seconds to .15 seconds. This doesn't make you block any faster, just recover from a hit.
As far as "why my armour gets hit to avoid stun," you seem to have a misconception. Chance of being hit by a monster is determined by:
FTH = 30 + ToHit + 2·(mlvl-clvl) - Dex/5 - AC
Basically, your armor doesn't get hit, you do. Higher AC (Armor Class) means you get hit less. If you noticed a Ting! noise, that's you blocking. If you hear a Urgh! that's you getting hit, or stunned as you say which is the hit recovery I was referring to. Any other noise and the monster is missing you.
So suffixes of harmony ideal here? How do they work? i.e. if i have something of harmony on every piece of armour, can i never be stun blocked?
If i have say, just a harmony plate, does the game randomly decide when my armour gets hit to avoid stun, or chance of stun?
Harmony items do not stack. However, there is a particular build called the "Zen" build that mages sometimes use, and it involves using a Harmony item in conjunction with a Balance item and a Stability item. This three affixes DO stack, but you need 1 of each to make it work. If you're playing a warrior though, I wouldn't recommend using harmony if you have something better, as warrior's have the best hit recovery of the classes anyway. Unless you are playing something crazy, like a Low-Ac axer or staff warrior, than harmony maybe quite useful.