What items does each character most strongly benefit from?

This is a really broad, and somewhat opinion based question, but I'll try to answer it to some extent. I'll rate the item effectiveness per character for each item on this scale:

1(Bad) (has a detrimental effect) 2(Neutral) (has a low or non existent effect) 3(OK) (has an moderate level of effectiveness.) 4(Good) (is an above average choice for that character) 5(Great) (is much more useful for this character than for others)

Soldier's Syringe:

• Commando 4
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 2 (1st attack doesn't get much. Other attacks charge/hit slightly faster.)
• Mercenary 4

Tougher Times:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 4 (with the artificer being a glass cannon, defensive items are essential)
• Mercenary 3

Monster Tooth:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 4 (Monster tooth drops healing orbs on kills, making it well suited for melee attacks)

Lens-Maker's Glasses:

• Commando 4 (all classes benefit greatly from having up to 10 lens-maker's glasses.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Paul's Goat Hoof:

• Commando 3 (all classes benefit from having a few goat hooves)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Bustling Fungus:

• Commando 2 (for most classes, bustling fungus is only good for healing from blood shrines)
• MUL-T 2
• Huntress 2
• Engineer 5 (the engineer's turrets benefit greatly from the bustling fungus.)
• Artificer 2
• Mercenary 2

Crowbar:

• Commando 3
• MUL-T 4 (High damage single attacks, like the Rebar Puncher benefit most from the Crowbar)
• Huntress 3
• Engineer 3
• Artificer 4 (High damage attacks are made much more effective with the crowbar.)
• Mercenary 3

Tri-Tip Dagger:

• Commando 3 (all classes benefit from the increased dps of the tri-tip dagger.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Warbanner:

• Commando 2 (the Warbanner is situational and loses effectiveness at higher levels.)
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 2
• Mercenary 2

Cautious Slug:

• Commando 2 (the cautious slug is a mildly effective source of healing.)
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 2
• Mercenary 2

Personal Shield Generator:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 4 (with the artificer being a glass cannon, defensive items are essential)
• Mercenary 3

Medkit:

• Commando 2 (the medkit is a mildly effective source of healing.)
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 2
• Mercenary 2

Gasoline:

• Commando 3
• MUL-T 4 (Being able to 1-shot weak enemies makes a few stacks of gasoline very effective)
• Huntress 3
• Engineer 3
• Artificer 4 (Being able to 1-shot weak enemies makes a few stacks of gasoline very effective)
• Mercenary 3

Stun Grenade:

• Commando 3 (The stun grenade is effective, but works best with a stack of 20.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Bundle of Fireworks:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 4 (Fireworks provide a ranged option for the mercenary.)

Energy Drink:

• Commando 3
• MUL-T 4 (Transport Mode is much more effective with several speed items.)
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Backup Magazine:

• Commando 3
• MUL-T 4 (Blast canister is more powerful with a few stacks of the Backup Magazine.)
• Huntress 4 (Laser Glaive is more powerful with a few stacks of the Backup Magazine.)
• Engineer 2 (Pressure Mines already stacks to 10, and can't have more than 10 placed at a time.)
• Artificer 4 (Nano-Bomb is more powerful with a few stacks of the Backup Magazine.)
• Mercenary 4 (Whirlwind is more powerful with a few stacks of the Backup Magazine.)

Sticky Bomb:

• Commando 4 (High stacks (up to 40) of sticky bombs are devastating with any character.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Rusted Key:

• Commando 2 (Extra stacks result in rarer items, though it has no other benefit.)
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 2
• Mercenary 2

Armor Piercing Rounds:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 4 (As the artificer is already good at boss killing, these amplify that power.)
• Mercenary 3

Atg Missile Mk1:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 4 (Atg missiles provide a ranged attack option.)

Will-o'-the-Wisp:

• Commando 3
• MUL-T 4 (Being able to 1-shot weak enemies makes a few stacks of Wisps very effective)
• Huntress 3
• Engineer 3
• Artificer 4 (Being able to 1-shot weak enemies makes a few stacks of Wisps very effective)
• Mercenary 3

Hopoo Feather:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 4 (a few stacks of feathers compensate for low mobility and speed.)
• Artificer 4 (a few stacks of feathers compensate for low mobility and speed.)
• Mercenary 3

Ukulele:

• Commando 3 (The ukulele makes any character better at crowd control)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Leeching Seeds:

• Commando 3 (The leeching seeds provide moderate healing while attacking.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Predatory Instincts:

• Commando 4
• MUL-T 4
• Huntress 4
• Engineer 3 (the turrets benefit more from this item than the engineer himself does.)
• Artificer 2 (attack speed items are not very effective with the artificer.)
• Mercenary 4

Red Whip:

• Commando 3 (increased speed out of combat is good for exploration.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Old War Stealth Kit:

• Commando 2
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 3 (this item provides unreliable stealth and defensive ability.)
• Mercenary 2

Harvester's Scythe:

• Commando 3 (more effective than leeching seeds IF you have 10 lensmaker's glasses.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Fuel Cell:

• Commando 4
• MUL-T 5 (this item is twice as effective on MUL-T as he carries 2 sets of equipment.)
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Infusion:

• Commando 4 (all characters benefit greatly from the extra hp that infusion provides.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 5
• Mercenary 4

Bandolier:

• Commando 3
• MUL-T 4 (running enemies over in transport mode is easier with this item.)
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 5 (melee range suits this item well.)

Berzerker's Pauldron:

• Commando 4
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 3 (the artificer does not benefit very much from attack speed items.)
• Mercenary 4

Rose Buckler:

• Commando 2
• MUL-T 4 (MUL-T is often using sprint mode after using transport mode.)
• Huntress 3 (The primary ability can still be used while sprinting, making this item better.)
• Engineer 2
• Artificer 2
• Mercenary 2

Runald's Band:

• Commando 3 (all characters benefit from the chance of extra damage this item provides.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Kjaro's Band:

• Commando 3 (all characters benefit from the chance of extra damage this item provides.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Chronobauble:

• Commando 2 (this item is a bit underwhelming compared to other green items.)
• MUL-T 2
• Huntress 2
• Engineer 2
• Artificer 2
• Mercenary 2

Wax Quail:

• Commando 2
• MUL-T 2
• Huntress 2
• Engineer 3 (a few stacks of wax quails compensate for low mobility and speed.)
• Artificer 3 (a few stacks of wax quails compensate for low mobility and speed.)
• Mercenary 2

Brilliant Behemoth:

• Commando 4 (all characters benefit from increased area effect damage.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Ceremonial Dagger:

• Commando 4 (all characters benefit from increased damage on kill from dagger projectiles.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Frost Relic:

• Commando 3
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 5 (the mercenary in particular is well suited to this melee range attack item.)

Happiest Mask:

• Commando 4 (having extra allies distributes enemy damage more evenly.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

H3AD-ST v2:

• Commando 4 (all characters benefit from increased jump height and a special attack.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

N'kuhana's Opinion:

• Commando 4
• MUL-T 4
• Huntress 4
• Engineer 5 (super effective when combined with the bustling fungus as the engineer.)
• Artificer 4
• Mercenary 4

57 Leaf Clover:

• Commando 4 (amplifies the chances of all proc items.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Sentient Meat Hook:

• Commando 4 (good for pulling enemies into clusters.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Alien Head:

• Commando 4 (cooldown reduction helps for all classes)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Soulbound Catalyst:

• Commando 4
• MUL-T 5 (faster use item cooldowns are even more effective for MUL-T)
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Dio's Best Friend:

• Commando 3 (The extra life is nice, but does little for dps or long term survival ability.)
• MUL-T 3
• Huntress 3
• Engineer 4 (these are re-usable when applied to the turrets.)
• Artificer 3
• Mercenary 3

Hardlight Afterburner:

• Commando 2 (commando gains less benefit from this item than the other classes.)
• MUL-T 4
• Huntress 3
• Engineer 4
• Artificer 3
• Mercenary 4

Wake of Vultures:

• Commando 3 (particularly useful for activating blood shrines after killing shield elites)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

Brainstalks:

• Commando 4
• MUL-T 4
• Huntress 4
• Engineer 2 (the engineer benefits less from this item.)
• Artificer 4
• Mercenary 4

Rejuvenation Rack:

• Commando 4 (all characters benefit from the increased healing this item provides.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Titanic Knurl:

• Commando 4 (all characters benefit from the hp and regen bonus this item provides.)
• MUL-T 4
• Huntress 4
• Engineer 4
• Artificer 4
• Mercenary 4

Queens Gland:

• Commando 3 (the insect can be mistaken for an enemy, but does draw fire away from you.)
• MUL-T 3
• Huntress 3
• Engineer 3
• Artificer 3
• Mercenary 3

As a disclaimer, this list is mostly opinion based, and highly subjective. This takes into account the base effects of these items, but some of them are more or less effective in certain sets. This does not include any information on Equipment/Use items or Lunar items.