Hello, this is Grathox from League of Legends, and this is my Garen Build. This build will show how to be a great tank, and still do a decent amount of damage.

I've gone with 9 Greater Mark of Strength, 9 Greater Seal of Resilience, 9 Greater Glyph of Warding and 3 Greater Quintessence of Desolation so that you have some armor penetration, Attack Damage and some extra armor + magic resist in early game. You could swap out some of the Attack Damage Marks for some Armor Penetration marks, but I find the Attack Damage more viable to give you a boost in early game (which will probably give you the upper hand late game).

I've chosen Ghost and Ignite as summoner spells, since Garen doesn't really need any other as much as those. Ghost can be used to catch up with your enemies and Ignite to attempt to kill them if they go behind turret with low hitpoints and you can't follow.

Some other viable Summoner Spells would probably be Exhaust, Heal (In rare cases) and Flash if you need a quick get-away like eg. flashing through a wall.

I've taken 14 points in Offense, 14 points in Defense and 2 points in Utility for the best late-game and early-game performance (according to me, at least). I've tried out many masteries and this is the one I like the most, since the early-on attack damage and armor + magic resist really helps in those tough spots. The extra health will of course give you the upper hand over your enemies both late and early (mostly early).

I pick these skills for a balanced farming / killing early game. Your Judgement skill will really help you wound your enemies alot and finish off those minions, and your Decisive Strike will disrupt their spells and help you catch up to your enemies, and flee. I only pick one point in Courage early, and max it out later, since all you really need from it is the bonus armor and magic resist. Your ulti is your greatest source of damage and should be used to help your team or to finish off your enemies while they flee or it's an even between you and your enemy.

I've picked these items for a high damage output, and still a very good tank, the potions early really helps you to lane for a longer time (since life steal isn't that good for Garen), and the boots of speed and when they're upgraded to Boots of Ludicity is very good to catch up to your enemies and still reduce the cooldowns of your abilities, since Garen is only restrained by his cooldown, this is very good for him. As soon as you get Frozen Mallet you will become unstoppable (probably, not certainly). Once you get your Atma's Impaler your Attack Damage will shoot through the roof! And if that's not good enough, you will get a decent boost to your Armor! After that you will get Warmog's Armor, which will give you a huge boost (maxed or not) to your Health Reg, Health and Damage (if you have Atma's Impaler, but since you're following this build I'm guessing that you have it). The Thornmail after Warmog's will give you the armor and damage returned enough to be able to go 1v1 with ANY physical-based character without the need to worry, even a little bit. After that it's up to you, you could take Guardian Angel or Force of Nature depending on the opposite team and what you like the best, most of the times I go with Guardian Angel because the extra health reg and magic resist doesn't matter that much to me late game, but if your opponents consist mostly (or only) of spellcasters, your magic resist will be a great addition to the build.

Pros:
High Damage Output both Early and Late Game
Great Tank
A good team player, hard to kill
Great 1v1 character
Excellent farmer with your E (Judgement)
Great diver & Great and destroying their defense
Great at escaping or catching up
Good Solo Top, Top with 2 players and he's great when playing alongside with a Support

Cons:
Needs to farm hard or be fed
Long Cooldowns

Garen is a great farmer. Just use your E and auto attack between cooldown and the money will flow!

Garen is a great team player. He delivers a high amount of damage while taking basically little or none damage at all, even when being focused. He can save his team-mates by silencing the opponents or slowing them when he has Frozen Mallet.

I'd say that this guide is better than other builds, since this build is great for tanking and still being a great damage dealer, which makes you a huge threat towards the opposite team. When you have your build up you can easily 1v3 your opponents, since your Judgement (if properly used) will kill them or take them down to low hitpoints. My opinion is that Garen is a better tank than other tanks because of his high health, armor and his magic resist, and still able to kill almost everyone 1v1 (everyone if you're good). Using this build will be counter everyone, since you can easily counter jungle by stealing jungle or preventing spawn, counter all mage classes, or any class that relies upon magic by slowing them with Frozen Mallet and using your Decisive Strike to silence them, and popping your Judgement after to kill them (or using your Demacian Justice to execute them if your Q and E isn't enough).

The attack damage shown at the top of the page and the armor + Magic Resist is wrong. He actually has 2% of his max HP in attack damage and 25+ Armor and Magic Resist when Courage is maxed, additionally he will have more Health and Health Reg when Warmogs is maxed.

Any Questions about the build? I'm happy to answer them! I'm also open to suggestions about how to improve this build for certain things, just comment and write why you think that you do and how I should improve it, and I'll try your opinion out in game and decide if It's worth having or not :)