Everybody knows that debuffs in classic are precious, so I thought I'd put together a list and open it up for discussion whether I've missed something or which debuff is more valuable or if a particular class/spec is better at applying the debuff.
Debuffs use a priority system, where lower priority debuffs will be dropped if the debuff cap is reached. For Classic the debuff cap is 16 debuffs. CC/Taunt effects have the highest priority. Curses/Sunder/Thunderclap type abilities have the second priority. Vulnerability type debuffs(like winter's chill, shadowweaving) have third and DoTs have the lowest priority.

0 = ammo 1 = head 2 = neck 3 = shoulder 4 = shirt 5 = chest 6 = belt 7 = legs 8 = feet 9 = wrist 10 = gloves 11 = finger 1 12 = finger 2 13 = trinket 1. A simple guide on how to set up and organize Action Bars for the Warrior Class in Classic WoW.Background music by Justin Tiberius Venters:https://soundcloud.

Armor reduction debuffs:
Curse of Recklessness: Warlock #3, depending on how the final armor values shake out, we may only need this curse on Warrior type bosses
Sunder Armor: Protection Warrior #1, Pro-tip: every Warrior hits Sunder Armor once at the start of the fight to stack up to 5 instantly.
Expose Armor: Rogue, however does not stack with Sunder Armor. If talented, a 5 point Expose will reduce armor by 300 more than a full stack of Sunder Armor.
Annihilator: Warrior(s) Pro-tip: multiple dps/off-tanks with Annihilator to stack up, then swap to their regular weapons except for 1 to keep up the debuff. Lasts 45 seconds.
Faerie Fire: Druid, easiest applied by a feral druid off-tank using Feral Faerie Fire.

Magic damage increasing debuffs:
Curse of Shadow: Warlock #1
Curse of Elements: Warlock #2
Winter's Chill: Frost Mage
Shadow Weaving: Shadow Priest, however a Powerweaving(heavy Disc but enough for shadowweaving) or Holyweaving(heavy Holy, but enough for shadowweaving) Priest can fill the role as well. Typically Shadowweaving will take up additional debuffs slots from Shadow Word: Pain and Mind Flay, although once it is stacked up to 5 it can be easily kept up with Mind Blast on cooldown, say from Power/Holy weaving spec.
Nightfall: Phase 3, Off-tank Warrior is considered the best wielder of Nightfall because that gives them something to do when not tanking. Enhance Shaman/Ret Paladin/Wing Clip Hunter may have better uptime but these specs tend to be low priority for gearing.
Improved Shadow Bolt: Warlock, preferably several to keep the debuff rolling.
Improved Scorch: Mage, though we shouldn't see this until later phases as most of the bosses in MC are very resistant to fire damage.
Flame Buffet: Arcanite Dragonling, though we shouldn't see this until later phases as most of the bosses in MC are very resistant to fire damage.
Judgement of the Crusader: Paladin increases holy damage. However, unless you have a large number of Smite Priests it is not worth the debuff slot.

Ranged Attack Power
Hunter's Mark: Hunter, can be talented to increase the attack power. Since this debuff really only affects Hunters, unless you have quite a few in the raid it should probably be skipped.
Expose Weakness: Hunter from the 8 piece bonus of T2. It doesn't last very long and only affects Hunters.

Physical Damage
Hemorrhage: Subtlety Rogue, however does not stack with Gift of Arthas and has limited charges. Replace that rogue with a gift from our lord and savior, Arthas. I may have mained a Death Knight...
Gift of Arthas: Main Tank, this is effectively adding a Dense Sharpening Stone to everyone in the raid. Assuming that a hunter with a 3.0 weapon uses a pet with a 1.5 attack speed, this is better than Hunter's Mark even for the hunter. Hunter pets, Rogues and Feral Druids get the most benefit out of this debuff due to attack speed.

Mitigation
Thunderclap: Warrior, since this is only usable in Battle Stance, either an off-tank or an Arms warrior should keep this active. The main tank can do it in a pinch, but it requires stance-dancing.
Thunderfury: Warrior, this does not stack with Thunderclap, but is better anyway.
Demoralizing Shout/Demoralizing Roar: Warrior, because Demoralizing Shout is a larger reduction in attack power, though both can be talented to increase the amount.
Curse of Weakness: Warlock... #4?, reduces the damage of the boss by 31, though can be talented for an additional 20% and further amplified by 50% from Amplify Curse. Reducing a boss' damage by 55 damage may not sound like much, but that is roughly the equivalent to 780 attack power. However, very likely to be skipped.
Insect Swarm: Druid, while it does reduce the chance of the boss hitting by 2% it is likely not worth the debuff slot.

Notable Miscellaneous
Ignite: Mage, though we shouldn't see this until later phases as most of the bosses in MC are very resistant to fire damage. Ignite may buck the trend and actually be on the Vulnerabilities priority.
Judgement of Light: Paladin, melee attacks against the target heal for a small amount.
Judgement of Wisdom: Paladin, attacks and spells against the target have a chance to return 59 mana.
Vindication/Scorpid Sting: Ret Paladin/Hunter, it is unclear how raid bosses are affected by Strength/Agility losses.
Vampiric Embrace: Shadow Priest, however to get the most of the healing you would need to allocate 4 debuffs to that shadow priest alone.

So what am I missing other than explicitly listing out the best 16 debuffs?

'Hands' redirects here. For the hozen, see Hands (hozen).

• 1Armor
• 2Non-armor
• 3Weapons
• 4Decorative
• 5Other
• 6Removed

There are 18 equipment slots on a character, as well as 4 bag slots above the Micro Menu and a mount equipment slot in the Mount Journal. The combination of equipment slot and item type is often the first categorization of items used.

Armor

The eight armor slots may have one of four types of armor: cloth, leather, mail, and plate. Each class may equip only certain types of armor.

Head

Equipment names

helm, helmet, mask, lens, monocle, cap, headpiece, crown, circlet, shroud, cowl, hood, headband, goggles, coif, headdress, chapeau, barbute

Upgrades

Meta gems, Gems

Shoulders

Equipment names

amice, spaulders, pads, pauldrons, mantle, monnions, epaulets, shoulder, shoulderpads, shoulderguards

Upgrades

Shoulders enchants, Gems, Inscriptions (inscribers only)

Chest

Equipment names

robes, vest, tunic, armor, wraps, harness, hauberk, jerkin, chain, breastplate, blouse, chestpiece

Upgrades

Chest enchants and armor kits, Gems

Wrist

Equipment names

bindings, armguards, cuffs, bracers, vambraces, wristguards, bracelets

Upgrades

Wrist enchants, Gems, Fur Lining (Leatherworkers only), [Socket Bracer] (Blacksmiths only)

Hands

Slot Number Wow Classic Alterac

Equipment names

gloves, gauntlets, mitts, mittens

Upgrades

Hands enchants and armor kits, Gems, Tinkers (Engineers only), [Socket Gloves] (Blacksmiths only)

Waist

Equipment names

belt, girdle, sash, cinch, cord

Upgrades

Belt Buckles, Gems, Tinkers (Engineers only)

Legs

'Legs' redirects here. For the companion, see [Legs].

Equipment names

chausses, trousers, breeches, leggings, kilt, dungarees, loincloth, woollies, pants, britches

Upgrades

Legs armor kits and spellthreads, Gems

Feet

Equipment names

boots, walkers, greaves, footwraps, sabatons, slippers, shoes, stompers, footpads

Upgrades

Feet enchants and armor kits, Gems

Non-armor

Non-armor gear only has one type, so there will be nothing in the 'type' spot in item tooltips.

Neck

Equipment names

amulet, lei, talisman, medallion, necklace, charm, choker, chain, pendant, collar.

Upgrades

Gems

Back

Equipment names

cape, blanket, cloak, drape, mantle

Upgrades

Back enchants, Gems, Embroideries (Tailors only), Tinkers (Engineers only)

Finger

Equipment names

ring, band, signet, seal, loop, circle.

Upgrades

Enchantments (Enchanters only), Gems

Two finger slots are available

Trinket

Upgrades

Gems (Jewelcrafters only)

Two trinket slots are available

Weapons

Main-Hand (One-Hand/Two-Hand)

Weapon types

Axe, Bow, Crossbow, Dagger, Fist weapon, Gun, Mace, Polearm, Staff, Sword, Wand, Fishing Pole

Upgrades

Weapon enchants, Counterweights, Weapon Chains, Fishing line (fishing pole only)

The Main Hand slot can hold main hand, one-handed or two-handed weapons. If a two-handed weapon is equipped, the off-hand slot must be empty unless the character is a warrior with the [Titan's Grip] talent.

Slot Numbers Wow Classic

Off-Hand (One-Hand/Two-Hand)

Weapon types

Axe, Dagger, Held In Off-hand, Fist weapon, Mace, Shield, Sword

Upgrades

Enchantments, Counterweights, Weapon Chains, Shield Spike

The Off Hand slot can hold off hand or one-handed weapons (Dual wielding classes only), shields (warrior, paladin and shaman only) or Held In Off-hand frill items. If a two-handed weapon is equipped in the main hand slot, this slot must be empty unless the character is a Fury warrior with [Titan's Grip].

Decorative

Decorative items only have one type, so there will be nothing in the 'type' spot in item tooltips. Decorative items have no stats and cannot be upgraded.

Tabard

Shirt

Other

Bag

Removed

Slot Number Wow Classic Mithril

Ammo

Ranged or Relic

Slot Number Wow Classic Addon

Equipment slots in macros

When creating macros based on 'click to use' items (especially trinkets), the common practice is to enter the actual item name. However, using the numeric equipment slot identifiers eliminates the need to manually modify macros after upgrading a particular item.

These slot designations are particularly handy for engineers whose Tinker enhancements and Engineering-only items add and/or posses an 'on use' ability for items in the Back, Belt, Foot and Head slot that would not normally posses such capability for non-engineers.

Here are is a complete list of each equipment slot and its numeric designation for macro purposes

Patch changes

• Patch 5.0.4 (2012-08-28): Range/Relic slots are being removed from all classes. Range weapons (bows, guns, wands) will use the main hand slot.
  
• Patch 4.0.1 (2010-10-12): The ammo mechanic was removed from the game, and the ammo slot was removed along with changing ammo items to gray items.
  

Slot Number Wow Classic Wow