Garen

Demacian Justice for All

Efficient early lane trades and kill secure with [R].

Durability and crowd control

Grasp of the Undying

Every 4s your next attack on a champion deals bonus damage, heals you, and permanently increases your health.
Demolish

Charge up a powerful attack against a tower while near it.
Conditioning

After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Unflinching

Increase your Tenacity and Slow Resistance for each summoner spell on cooldown. Immediately after casting a Summoner Spell, gain a larger burst of Tenacity and Slow Resistance for a moderate duration.

Improved attacks and sustained damage

Triumph

Takedowns restore health and grant bonus gold.
Coup De Grace

Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.

Unfortunately Garen doesn't have any fantastic keystone choices, Grasp is just the best of the bad.

You should be able to proc Grasp relatively easily in melee matchups with [Q] and the bonus max health synergizes well with your [W].

Demolish on Garen allows you to massively chunk towers when combined with the empowered auto from [Q].

Conditioning allows you to scale into the mid game as a very threatening tank.

Strong AD Matchups: If playing into a strong AD matchup like Renekton or Camille, take Iron Skin.

Strong AP Matchups: If playing into a strong AP matchup like Rumble, take Mirror Shell.

The bonus tenacity and slow resistance will help you overcome Garen's biggest pitfall of getting kited by CC and slows.

With Unflinching you should be able to stick to your target for the kill much more easily.

Triumph on Garen allows you to absorb a lot of frontline damage by picking up kills/assists during teamfights.

Coup De Grace's damage amplification synergizes well with Garen's execute oriented [R] for that last little bit of kill potential.