We nezmodders have seen them. We know what they are. We know what to do with crap souls. But... what is really defined as crap souls.
USually crap souls are souls that have powers that you don't really want to benefit off for late game. If you just started and you found it, there can be quite a difference.
This list of monsters are the recommended souls you should watch out for and why you should keep them. A picture or a visual aid is provided in case you are new and don't know what monster we are talking about.
To use them, simply equip them to the Ring Slots. You could also get a Montreal's Spirit Jewel device from Mystery Chests or Gauntlet. With this, a soul and 2 space frags, you create a Jewel you can place into sockets.
People must PM me with thier recommended reason for this soul, a visual of the creature if its new, where they could appear, and what powers the provide.
Note that some of the powers have +X on some places. This is because it changes depending on the difficulty you collected the soul and what class you are.
This list will grow as time goes on.
Monster #1 - Baal's Minion Type
Varieties - Enslaved, Slayer, Ice Boar, Fire Boar, Ice Spawn, Hell Spawn, Greater Hell Spawn, Greater Ice Spawn
Locations - Starts appearing in Act 4 City of the Damned/Chaos Sancturary (Enslaved w/ Overseer) to Act 5 Worldstone Keep (Greater Hell Spawn)
Soul Powers - IAS, Faster Block Rate, +x to Sword/Axe/Mace Masteries with another bonus if you are a barbarian (Barbarian will reduce the all class +x and reduce the level requirement), Mana Leech, random elemental damage, +x defense vs melee attacks.
This monster is the bomb for melee classes. The breed of monster is quite easy to find too. Just remember that they appear with Overseers as well, and they can go kamikaze and you cannot get the soul if this happens.
The reason you want this soul is mainly for the Masteries, mana leech and ias. Something that melee characters love to benefit off of. With Masteries, you get a chance to criticial hit something AND you also get ED. These also benefit from other +x all skill items.
Use this soul with Paladins and Barbarians that do heavy melee combat. Zealers can also benefit from this, since the most common Chaos|Depression's Triumph weapon people use is the one handed Berserker Axe (Axe mastery is on this soul).
Monster #2 - Izual
Varieties - Izual, Fallen Angel
Locations - Plains of Despair (Izual), Kagero Level 4 and 5 (Fallen Angel)
Soul Powers - X% chance of Level X First of Heavens on Striking, Level X Conviction Aura when Equipped, +X to Summon Fallen Angel, +X to Frost Nova, another bonus to this if you are a Sorceress, +X Defense Vs Missiles, Resist all (extra to cold), Damage Reduction, Cold Damage
A badass soul for a basic Conviction Aura and some Damage reduction, but that is not all its worth for. This soul allows you to summon Izual himself to aid you in combat.
He fights the same way except loses Frost Nova. He also has a chance of casting Fist of Heavens on striking, which will do more damage if the enemy is under conviction and not immune to lightning. He has plenty of hit points when you get him and does some decent damage. You will need +X skill items to improve this power. Once you got that through, he is one of the ultimate tank summons in the game (Next to Golems, Bears and Marilith).
I do believe that Izaul's variation is roughly the same power as the Fallen Angel. Getting them is a balance. Izual only appears once but any character around his level or greater can kill him. Fallen Angels are more common in Kagero but you need to survive the other monsters. The Fallen Angel has less HP than the Izual counter part. It is also immune to Lightning and Cold in Hell.
Use this soul with Summon Necs, Summoning Druids or other classes that need a spare meat shield to assist you.
Monster #3 - Doom Knight

Note: This is the Abyss Knight. The Doom Knight doesnt have a glowing right hand.
Varieties - Doom Knight, Nezeramon Guard
Locations - Anywhere in Act 4 except River of Flame and City of the Damned (Doom Knight, appears with Oblivion Knights in Chaos Sanctuary), Level 1 Kagero (Nezeramon fighter with Omega Knights), Act 5 Throne of Destruction and Halls of Pain (Appears with Baal's Subject Mummy)
Soul Powers - IAS, Elemental Damage of all kinds (Spectral Hit passive), +X Summon Doom Knight, +X Skeleton Mastery, Resist All elements except magic.
The only thing good about this soul is the +X Summon Doom Knight power. This skill destroys a corpse and calls an A4 Doom Knight from it. It will fight with a sword and slash any enemy it finds. Much like the Skeleton does.
What's the difference? At level 1, a Doom Knight has around the same DEfense and Attack Rating as a Skeleton and you can only summon 1. However, the Doom Knight has over 260 Life in Normal and does 14-25. A normal skeleton as like 35 Life and only does 1-2. It also recieves bonuses from Skeleton Mastery, like Skeletons do. It costs around the same mana to call a Doom Knight than a Skeleton.
Early on, you can use this to make your summon army stand up to combat better till you can use Marilith, better Golems, Fallen Angel or other better summons. Late game, they can still flood the area like Skeletons and Mages could... just you need +X all skill items to improve thier powers.
Use this mostly with a Summon Necro if you are looking for a Quantity type of summoner (Goblins, Hell Cats, Crows, Flesh Worms). If you are searching for Quality, you could still use them if you want but you would probably be better off with Skeletons, Golems or other summons. But either type of summoner still hurts like hell.
Monster #4 - The Smith
Varieties - The Smith, Moe, Larry, Curley, Hephasto The Armorer
Locations - Act 1 Barracks (Smith), Pit Level 2 (Stooges). Act 4 River of Flame Hellforge (Hephasto)
Soul Powers - X% chance to cast Level X Amplify Damage on Striking, Level X Concentration Aura when Equipped, IAS, Faster Hit Recovery, various elemental damages, -X% resist drop on enemy, X% Crushing Blow, Prevent Monster Heal, Fire Resist, drops Poison Resist
The poison resist drop is the only drawback to this soul, which is easy to fix with PSN resist items or length reducers. The amp damage on striking and Concentration Aura makes it a good reason to collect this soul, especially in team games or with summons. This is the only way to get this aura except thru Malarith. The resist drop is small, but it could give you the small edge you need to combat a foe.
Note that Smiths have the aura when you fight them, so be prepared to take some damage if your character is a little weak.
This aura also works on magic spells, but at half capacity.
Use this with melee characters or Summon characters for maximum effects. Just make sure you counter the poison drawback.
Monster #5 - Vile Mother
Varieties: Flesh Spawner, Stygian Hag, Grotesque
Locations: Act 4 Outer Steppes/Plains of Despair (Flesh Spawner), City of the Damned (Stygian Hag), River of Flame (Grotesque)
Soul Powers: Poison Damage, +X to Summon Vile Child, +X% Maximum Life, +X% Slower Stamina Drain, +X% Shorter Curse Duration, +X% Cold Resist, +X% Shorter Poison Length
Summoners will love this one! Not only it provides you with a good life and status debuff duration decrease, it allows you to add another summon to your roster, the Vile Children. Vile Children are little worms that will scratch at any monsters that attack them. They also have an innate life steal on thier attacks, so they can heal themselves as they strike.
This summon like the Doom Knights, Hell Cats and Skeletons are quantity over quality. With good buffs and healing going off, these summons can share the valor and strength of any skeleton you summon.
They are best used with Summon necs, druids or other summon classes. Just be sure to use +X skill items to improve this skill.