Hi my name is Ruben. I am a Garen main that which plays on EUW. Garen is my favorite champion, and in my opinion he is one of the strongest top laners.

Pros

Strong early game
Good Tank
Long Silence
Able to remove slows with Q
Easy farming
Ultimate scales with the others
missing hp=tons of damage
No mana/fury/health need to use abilities
Good diver
Good counter to the meta
Able to be a beyblade with E

Cons

Falls a bit off late
Bad initiate
Only 1 escape tool
E slows you down
Damage is dramatically reduced late game if you build tank
Ult scale magic pen

MARKS - Armor Pen: Armor pen is fantastic on Garen since then it will be a lot easier to deal damage with your E throughout the game

Seals - Armor flat: Flat armor is great on Garen, since you mostly meet AD top this will help you take more damage from them and minions

Gylphs - Magic resist scaling: Scaling MR (magic resist) are fantastic runes on Garen. Since it will scale better into to late game you wont be as strong as AP top, but most top laners are AD.

Quints - Movement speed: Movement speed scales well with Garen since your Q stacks with %-movement speed. Also since Garen has no movement speed/ jump/ teleport without his Q or flash(or teleport if you have) some extra movement speed are always usefull.

I think mallet is a perfect item for Garen since his E + Q combo will be much easier to pull off. often you will also nearly hit your whole e, there for i think mallet is the perfect item for Garen. It grants HP, AD and the passive which to me is kind of OP

Farming is easy
Farming is pretty easy with Garen, because since you have no mana-ish resource you can use your abilities to farm. Also the E is fantastic after level 3 to clare the minion wave. After you get sunfires cape (usually 1. or 2. item) your farming will reach a new level of easiness.

After all Garen is a fantastic top lanes that counter the meta today. His potential to dive is also strong with a executioner ult, silence, and damage reduction with W. If you get randuins or have a teammate with a strong initiate Garen has an easy time closing the gap and coming in tanking and dealing damage. Otherwise you get the oppertunity to become a living beyblade how can this guy be any more awesome. be sure to pick him up :D