
Marksman · physical (AD) damage
Jayce is a Marksman that deals physical (AD) damage and brings airborne. Some of their lockdown (airborne) ignores tenacity, so you dodge it; you can't cleanse or Merc' through it. Below is Jayce's highest-win-rate build from high-elo games; open the counter-build tool to tune it against your specific enemy team.

When Jayce swaps weapons he gains Move Speed for a short duration.

Hammer Stance: Leaps to an enemy dealing physical damage and slowing enemies. Cannon Stance: Fires an orb of electricity that detonates upon hitting an enemy (or reaching the end of its path) dealing physical damage to all enemies hit.

Hammer Stance: Passive: Restores Mana per strike. Active: Creates a field of lightning damaging nearby enemies for several seconds. Cannon Stance: Gains a burst of energy, increasing Attack Speed to maximum for several attacks.

Hammer Stance: Deals magic damage to an enemy and knocks them back a short distance. Cannon Stance: Deploys an Acceleration Gate increasing the Move Speed of all allied champions who pass through it. If Shock Blast is fired through the gate the missile speed, range, and damage will increase.

Hammer Stance: Transforms the Mercury Hammer into the Mercury Cannon gaining new abilities and increased range. The first attack in this form reduces the target's Armor and Magic Resist. Cannon Stance: Transforms the Mercury Cannon into the Mercury Hammer gaining new abilities and increasing Armor and Magic Resist. The first attack in this form deals additional magic damage.
E Thundering Blow / Acceleration GateHover a crowd-control type to see which ability it comes from.
Sorcery
Summon Aery
Arcane Comet
Deathfire Touch
Axiom Arcanist
Manaflow Band
Nimbus Cloak
Transcendence
Celerity
Absolute Focus
Scorch
Waterwalking
Gathering Storm
Inspiration
Glacial Augment
Unsealed Spellbook
First Strike
Hextech Flashtraption
Magical Footwear
Cash Back
Triple Tonic
Time Warp Tonic
Biscuit Delivery
Cosmic Insight
Approach Velocity
Jack Of All Trades

Based on 5400 games · 48.2% win rate · challenger+grandmaster+master+emerald-diamond+platinum-emerald · patch 16.18.1.
Inspiration
Glacial Augment
Unsealed Spellbook
First Strike
Hextech Flashtraption
Magical Footwear
Cash Back
Triple Tonic
Time Warp Tonic
Biscuit Delivery
Cosmic Insight
Approach Velocity
Jack Of All Trades
Precision
Press the Attack
Lethal Tempo
Fleet Footwork
Conqueror
Absorb Life
Triumph
Presence of Mind
Legend: Alacrity
Legend: Haste
Legend: Bloodline
Coup de Grace
Cut Down
Last Stand

Based on 1460 games · 43.8% win rate · challenger+grandmaster+master+emerald-diamond+platinum-emerald · patch 16.18.1.
Inspiration
Glacial Augment
Unsealed Spellbook
First Strike
Hextech Flashtraption
Magical Footwear
Cash Back
Triple Tonic
Time Warp Tonic
Biscuit Delivery
Cosmic Insight
Approach Velocity
Jack Of All Trades
Domination
Electrocute
Dark Harvest
Hail of Blades
Cheap Shot
Taste of Blood
Sudden Impact
Sixth Sense
Grisly Mementos
Deep Ward
Treasure Hunter
Relentless Hunter
Ultimate Hunter

Based on 1295 games · 52.1% win rate · challenger+grandmaster+master+emerald-diamond+platinum-emerald · patch 16.18.1.
Sorcery
Summon Aery
Arcane Comet
Deathfire Touch
Axiom Arcanist
Manaflow Band
Nimbus Cloak
Transcendence
Celerity
Absolute Focus
Scorch
Waterwalking
Gathering Storm
Inspiration
Glacial Augment
Unsealed Spellbook
First Strike
Hextech Flashtraption
Magical Footwear
Cash Back
Triple Tonic
Time Warp Tonic
Biscuit Delivery
Cosmic Insight
Approach Velocity
Jack Of All Trades

Based on 93 games · 43% win rate · challenger+grandmaster+master+emerald-diamond+platinum-emerald · patch 16.18.1.