It has always struck me as a bit of a dissappointment how greatly the potency of the Amrath sets fluctuate. Some are so good that all classes want them, or all classes of a specific type, while others are considered junk sets, or junk rings even to the players who have the prestige enhancement for the given set. What could be potentially 32 sets of equipment that would benefit various builds and roles are instead 4-5 high-powered sets and 25 junk sets.
Exorcist of the Silver Flame
This is a hard set bonus to comment on, since it is tied to the usefulness of the Turn Undead ability, which is very limited. Main way to make it useful would be to make TU and the variants thereof useful (DV/DH etc).
Hunter of the Dead
Suffers from a bit of the problem that many situational sets suffer from, that its often much more worthwhile to simple build your set around the Shintao set, which has hit/damage bonus that rivals even what the Hunter of the Dead set has.. against 1 type of enemy.
Defender of Siberys
The Defender of Siberys isn't as bad as some, main problem is probably that the set bonus is divided in a very weird way. It gives +15% melee hate, and a +2 AC bonus. Either the tank will be focused on high AC, using intimidate to keep agro, or he will be a pure HP/Hate tank, in which case AC rarely is high enough to be useful. I think many tanks would find these two sets alot more interesting if one was simply focused on hate tanking, and the other focused purely on AC/Intimidation tanking. Give the Defender of Siberys a +15% melee hate bonus, and any melee attack done by the character gains Lesser Vampirism.
Air Savant, Fire Savant, Water Savant, Earth Savant
All of them are inferior to Rakhir. One way to make the various elemental sets interesting to arcane casters would be to make the set bonus give a 10% discount to the SP cost of the affected spells in addition to their current effect (similar to Staff of the Perditioner, but not stacking with staff).
Stalwart Defender
See Defender of Siberys. Stalwart Defender set could be made the AC/Intimidation set, if Siberys is made the hate tank set. A +2 stacking bonus to intimidate as the base bonus, and a +2 AC and +1 shield block DR bonus as the bonus for characters with all Stalwart Defender III would make it a much more interesting set for intimidation tanks.
Deepwood Sniper
Not outright bad, situationally useful.
Purple Dragon Knight
At least this set doesn't suffer from the "confusion" of the two tanking sets, but just taking Shintao will often yield better results, for both damage and hate. A much more interesting set bonus would be, for example, +15% melee hate and Radiance Guard, making it a very useful epic and high-difficulty tanking set.
Dragonmark Heir
Suffers from the same problem as Exorcist, it is tied to the weakest feats in the game. Dragonmarked feats would need a significant boost for this set to be useful.
Frenzied Berserker
One the sets that just blows the others away by a wide margin.
Arcane Archer
Also one of the better sets, for not only arcane archers but any ranged-focused character.
Occult Slayer
One of the sets almost everyone wants, +6 Con and GFL belt, and a ring with +1 exceptional Con, and a set bonus that gives stacking bonus to saves.
Tempest
Again, one of the more useful sets, but mainly if you have Tempest III. Even with Tempest III, it only just barely beats Shintao by a small margin, and the Shintao ring gives +1 exceptional strength while the Tempest ring gives +1 exceptional wisdom.
Wild Mage
The main problem with this set bonus is that many casters are item-slot starved, having to waste 2 slots for a bonus they could easily get on just one item is a waste. Either the bonus would have to be stacking or better for the set to be worthwhile (either +2 to lvl 9 spells or less, or +3 to lvl 8 spells or less).
Shintao Monk
The melee set.
Knight of the Chalice
For anyone without KoTC, the Occult Slayer is much, much better, especially considering how easily SR is obtained in the game (any divine caster will trumph this set bonus with the click of a button). Even for a KoTC, the set is bland. To get the full benefit from Knight of the Chalice, it is often far better to be dual-wielding since the bonus damage from the set applies to each weapon hit, so more hits = more damage. This can lead to issues with hit, since you don't have as many feats as fighters, or the stable kensai bonus to hit as a fighter. If the set gave a +2 bonus to hit and 22 SR, and then additional damage against outsiders if you have KoTC III, then it would be far more interesting for a paladin.
Thief Acrobat
This set always really puzzled me. A Thief Acrobat will rarely have much problems with jump, especially since the belt has 3 jump clickies on it. Adding another +15 jump as a set bonus is incredibly redundant when jump is capped at 40. The proc is also very redundant, has a very low chance to proc and gives a much smaller AC penalty than destruction/improved destruction.. that work on every hit. A more appropriate and useful bonus would be to give the character Vertigo +10 (not stacking with the weapon ability).
Warpriest
Same problem as Exorcist, tied to a very weak ability. Even a battle-cleric would prefer Shintao.. because Shintao is just better.
Ninja Spy
While the Ninja Spy set isn't bad, every melee prefers having Tharnes Goggles which give a greater benefit, instead of having to blow two item spots (a problem many of the sets suffer from, that working two pieces of equipment into your gear for a set-bonus that is easily beaten by single piece of equipment). The threat reduction still means the set is situationally useful, if you're a high-DPS melee in a situation where you don't want agro, if you don't mind the loss in DPS from loosing the Shintao bonus. A much more appropriate bonus would be +2 to damage and 20% reduced threat, which would make the set nice for people who otherwise use Shintao, in situations where they want to decreate their threat.
Henshin Mystic
Again, a very situationally useful set, beaten by Shintao in 9 out of 10 cases (since the vast majority of outsiders are immune to fire). Useful in situations with fire-vulnerable mobs, otherwise its easier to just stick to Shintao. Seeing as you have no useful way to really dump ki, the set is rarely useful as you will have enough Ki as a monk to basically use Touch of Death whenever it is off cooldown even without the set, and even toss in the occasional lightning or void strike. The necklace is decent by itself before you acquire a ring to complete the set, at which point Shintao wins by a wide margin (especially since monks are typically a bit starved for +hit). As with the KoTC set, adding a +2 bonus to hit in addition to the 1d6 fire damage would move it closer to Shintao in desirability.
Radiant Servant
Again, an extremely situational set. There are very, very few undead opponents on level 18 and above, and those opponents are better handled with firewalls than a cleric using turn undead (abbot, as example). The additional Turn Undead attempts are not tempting either since, as mentioned, Turn Undead or the abilities based on it aren't terribly useful. A way to make the set, and in general clerics, more interesting would be if the set bonus was that each healing tick from Positive Energy Aura also restores 3d4+3 SP to anyone in the vicinity. Would also help with the main problem with the healing aura, that it requires the cleric to be next to the melee, which is where the cleric rarely wants to be unless he is built for it.
Archmage
Again, one of those sets that just trumphs the others sets of the same type.
Mechanic
This set bonus suffers from a problem that many sets have, they boost the strong suit of a prestige enhancement that already has more than plenty of what is being boosted. When a 18 Wizard/2 Rogue can handle traps on Epic difficulties, there is no real reason to bring a Mechanic. Any mechanic rogue that can't handle the hardest traps in the game already will have built his character horribly wrong already. Change the set bonus to a +15 bonus to Disarm/Open Lock, and an additional 2d6 bonus to damage with Light and Heavy Repeating Crossbows.
Ravager
Another one of the superior sets
Pale Master
A rather nice set, for pale masters.
Spell Singer
Not a terribly useful set bonus at all, the vast majority of bards have more than enough songs to keep people buffed throughout a raid or quest. Seeing as Spell Singer is getting some love next update, its hard to give a suggestion for an alternate bonus until the Prestige Enhancement is finished.
Virtuoso
Same problem as Spell Singer.
Kensai
Suffers from the same problem as the Mechanic set. The bonus to confirm critical hits is extremely redundant since a Kensai will already have an obscene bonus to confirm critical hits as is, especially since Critical Accuracy III is a requirement for Kensai III. You have a +6 bonus to confirm critical hits from those enhancements already, a +4 bonus to confirm critical hits with the weapons that you chose as your Kensai weapon, and all this for a prestige enhancement that is already one of the prestige enhancements with the highest +hit in the game! If the bonus was changed to give a +3 bonus to hit, and a +3 exceptional bonus to Trip, Sunder Armor and Stunning Blow DC's for characters with Kensai III, it would become a decent set for Kensai who like to take advantage of the increase in combat feat DC's from their prestige enhancement instead of another set in the large pile of sets that are "useful until you get Shintao".
Warchanter
A very peculiar set for a prestige enhancement that is typically used to create melee bards. If the set bonus instead gave a +2 bonus to hit (since bards are also typically hit-starved) and a +1 bonus to the damage bonus from your Inspire Courage song, I think more warchanters would consider the set.
Assassin
Actually the Assassin set is an example of a set that is fairly useful to its prestige enhancement without being so powerful that everyone wants it. I do though assume that the potency of the set depends greatly on your chance to apply level drain, which I'm not aware of.