Best in slot (BiS) items often depends on the boss you are up against. As a result it might be a good idea to keep different gear at hand for certain bosses. The list below contains the BiS items for mages in PvE, through the different content releases in WoW Classic. Not all items may be perfect for your build, but it should serve as a reference.

Phase 1 Pre-Raid

Slot Item
Head Green Lens of Frozen Wrath
Neck Tooth of Gnarr
Shoulder Boreal Mantle
Back Archivist Cape of Frozen Wrath
Chest Robe of the Archmage
Wrist Magiskull Cuffs
Hands Hands of Power
Waist Clutch of Andros
Legs Skyshroud Leggings
Feet Omnicast Boots
Ring Freezing Band
Ring Freezing Band
Trinket Briarwood Reed
Trinket Eye of the Beast
Main-Hand Inventor's Focal Sword
Off-Hand Drakestone
Two-Hand Elemental Mage Staff
Ranged Bonecreeper Stylus

Phase 2 Pre-Raid

Slot Item
Head Green Lens of Frozen Wrath
Neck Tempest Talisman
Shoulder Boreal Mantle
Back Archivist Cape of Frozen Wrath
Chest Robe of the Archmage
Wrist Sublime Wristguards
Hands Knight-Lieutenant's Silk Handwraps
Waist Clutch of Andros
Legs Skyshroud Leggings or Knight-Captain's Silk Legguards
Feet Knight-Lieutenant's Silk Walkers
Ring Freezing Band
Ring Freezing Band
Trinket Briarwood Reed
Trinket Eye of the Beast
Main-Hand Inventor's Focal Sword
Off-Hand Drakestone
Two-Hand Rod of the Ogre Magi
Ranged Bonecreeper Stylus

Phase 2

Slot Item
Head Netherwind Crown
Neck Choker of the Fire Lord
Shoulder Boreal Mantle or Lieutenant Commander's Silk Mantle
Back Sapphiron Drape
Chest Robe of the Archmage
Wrist Sublime Wristguards
Hands Knight-Lieutenant's Silk Gloves
Waist Mana Igniting Cord
Legs Netherwind Pants
Feet Knight-Lieutenant's Silk Boots
Ring Ring of Spell Power
Ring Ring of Spell Power
Trinket Briarwood Reed
Trinket Talisman of Ephemeral Power
Main-Hand Azuresong Mageblade
Off-Hand Drakestone
Two-Hand
Ranged Bonecreeper Stylus or Cold Snap

Phase 3

Slot Item
Head Mish'undare
Neck Choker of the Fire Lord
Shoulder Mantle of the Blackwing Cabal
Back Cloak of the Brood Lord
Chest Robe of the Archmage
Wrist Bracers of Arcane Accuracy
Hands Netherwind Gloves
Waist Mana Igniting Cord
Legs Netherwind Pants
Feet Ringo's Blizzard Boots
Ring Band of Forced Concentration
Ring Ring of Spell Power
Trinket Neltharion's Tear
Trinket Mind Quickening Gem
Main-Hand
Off-Hand
Two-Hand Staff of the Shadow Flame
Ranged Bonecreeper Stylus or Cold Snap

Phase 4

Slot Item
Head Mish'undare
Neck Choker of the Fire Lord
Shoulder Mantle of the Blackwing Cabal or Lieutenant Commander's Silk Mantle
Back Cloak of Consumption
Chest Bloodvine Vest
Wrist Bracers of Arcane Accuracy
Hands Netherwind Gloves
Waist Mana Igniting Cord
Legs Netherwind Pants
Feet Ringo's Blizzard Boots or Knight-Lieutenant's Silk Walkers
Ring Band of Forced Concentration
Ring Ring of Spell Power
Trinket Neltharion's Tear
Trinket Mind Quickening Gem
Main-Hand
Off-Hand
Two-Hand Staff of the Shadow Flame
Ranged Bonecreeper Stylus or Cold Snap

Phase 5

Slot Item
Head Mish'undare
Neck Amulet of Vek'nilash
Shoulder Mantle of the Blackwing Cabal or Lieutenant Commander's Silk Mantle
Back Cloak of the Devoured
Chest Robe of Volatile Power
Wrist Rockfury Bracers
Hands Dark Storm Gauntlets
Waist Eyestalk Waist Cord
Legs Leggings of the Black Blizzard
Feet Enigma Boots or Knight-Lieutenant's Silk Walkers
Ring Ritssyn's Ring of Chaos
Ring Ring of the Fallen God
Trinket Neltharion's Tear
Trinket Mind Quickening Gem
Main-Hand Sharpened Silithid Femur
Off-Hand Royal Scepter of Vek'lor
Two-Hand
Ranged Bonecreeper Stylus or Cold Snap

Phase 6

Slot Item
Head Frostfire Circlet
Neck Gem of Trapped Innocents
Shoulder Rime Covered Mantle
Back Cloak of the Necropolis
Chest Frostfire Robe
Wrist The Soul Harvester's Bindings
Hands Dark Storm Gauntlets
Waist Eyestalk Waist Cord
Legs Leggings of Polarity
Feet Frostfire Sandals
Ring Ring of the Eternal Flame
Ring Frostfire Ring
Trinket Neltharion's Tear
Trinket The Restrained Essence of Sapphiron
Main-Hand Wraith Blade
Off-Hand Sapphiron's Left Eye
Two-Hand Atiesh, Greatstaff of the Guardian
Ranged Doomfinger

Source