Indestructible Garen Tank Guide.
This builds toward huge survivability early / mid game. The options are to keep on stacking defensive stats or add more damage after your Core Build is finished.
This build is made for Summoner's Rift 5v5, i dont play other game modes.
I've done stats like 8:0:5, 7:1:6 for enemy teams to surrender around the 20 minute mark.
If you get some early kills, which is rather easy with this build, you will get incredibly durable and still hurt plenty.

Pros:

- Durable tank with good early game damage
- Hunt down and kill fleeing champions with super speed
- Bring Surprise Justice with Decisive Strike-Flash-Ignite-Ultimate combo
- Early/mid game domination
- Melee killer
- No farming needed early game
- Great farming mid game with Sunfire Cape / Judgement
- Can solo top except against double ranged

Cons:

- Weak against double ranged lane
- Low damage late game
- Cant jungle, cant solo dragon
- ezmode, nobody gives props for your awesome stats

Red Marks: Greater Mark of Strength: +0.95 Attack Damage
Yellow Seals: Greater Seal of Evasion: +0.75% Dodge
Blue Glyphs: Greater Glyph of Warding: +1.49 Magic Resist
Quintessences: Greater Quintessence of Swiftness: +1.5% Movement Speed

Damage helps you early game boosting your whirlwind to do some serious damage, provides the edge to get those early kills.
Dodge with Ninja Tabi and Masteries = always escape, frequent speed boost and serious AD migitation.
Magic Resistance because there are few good item choices for that, especially early game.
Speed because those 4.5% along with Force of Nature and dodge speed proc are what you need to hunt down most fleeing enemies and kite all over the map.

9:21:0

Full Tank with Cooldown / Magic Pen for Garen's Ultimate.

Dodge synergy with Ninja Tabi and Garen speed.

I decided for 21 points in Defense even though Ardor is kinda useless because the 4% reduction is a noticable difference when you take into account Garen's hp reg while more points in Offense dont really change the feeling of damage output but significantly reduce your tanking capability.

If you wonder why i skilled Exhaust when i dont even use it as a Summoner Spell.. well the other 2 options are just as useless for Garen.

I dont use the 9 points for support because besides the exp bonus the choices dont go for Garen. Especially since you dont die. Ever.

Flash and Ignite

Because this is how to use Garens Ultimate and finish off escaping champions or champions who think they are safe under a tower with a third of their hp left. Bring them Instant Justice:

Press Q, Flash to enemy, Decisive Strike with autoattack, Ignite and use Ultimate. Shield up if you are tower diving (I encourage to do this as soon as you hit level 6 ;)

If you are soloing top consider switching Ignite for Teleport to keep your tower guarded at all times.

You will use flash mostly to get in range of a candidate for Demacian Justice. However it works well for escaping too because you can usually make it to the next wall in whatever mess you've got yourself into.

Basic Items (Core Build):

Doran's Shield
Ninja Tabi
Sunfire Cape
Force of Nature

Doran's Shield is the obvious starting choice for tank Garen since its more hp reg and defensive stats. Ninja Tabi along with the suggested runes/masteries makes Garen almost immune to physical damage from early game on. Take Negatron Cloak for Force of Nature before Sunfire Cape if you are in trouble due to heavy AP enemy team. Usually though your Glyphs and Courage should bring you through ok untill Force of Nature after you finished the Sunfire Cape. It doesnt hurt to buy the base items Chain Vest, Negatron and especially Giant's Belt early and finish the Sunfire Cape / Force of Nature a little later if you have to go back to base for whatever reason i really cant imagine. Decide on what damage you are getting. Though you want to get that Sunfire as soon as possible to start farming. Last tank item i usually buy is Frozen Mallet, it provides all the Stats Garen loves and needs by now (Dmg, HP) along with a nasty slow effect. Warmog's armor can be picked too but i judge it inferior to the other choices, with an Atma's Impaler though it can provide some good stats for the buck.

Good Damage items are Last Whisper, Bloodthirster. Also Tiamat is an option for the hp reg and aoe effect, not my first choice though. If you are low on gold and really need some damage pick the Brutalizer, but leave it if things are working out already, the follow-up item is kinda meh for Garen, we are not interested in crit in this champion build. Sell the Doran's only if you need the space for a better item.

Priorities:

R - E - Q - W

Demacian Justice - Judgement - Decisive Strike - Courage

Exception: Take W (Courage) as 3rd skill after E and Q to max out your defense bonuses early.

Since you are very heavy from early game on you dont need to farm in early game. Go with a carry and let him farm to prepare for later game, take last hits only when your lane partner is going back to base. As soon as you have Sunfire Cape you can go farm waves ez mode with your aoe damage and awesome mobility. The bigger the wave the better. Don't forget your shield to minimize minion damage.

1. Ranged AD
2. Ranged AP
3. Melee
4. Support

Since Garen is a beast early game generally and especially with this build and doesnt need to farm you want to pick someone who can take advantage of this, aka a squishy ranged AD autoattack carry with huge range who can keep harrassing/farming and count on Garen to keep the enemies at a distance and scared to attack. Also Garen can interrupt a dangerous spell like for example Nunu's Ultimate with his quickly applied silence.

Caitlyn and Ashe are the best choices for this. Miss Fortune and Graves just a little behind.

The next best thing after ranged AD champs that can happen to Garen are ranged ap/ad champs with a low cooldown like Ezreal, Xerath, Twisted and the rest of them skill throwing champs.
They can harrass well, unfortunately they usually need mana and thus are limited at exploiting the enemys problem of not being able to attack at all because of the monster waiting in the brush.

Only after all the ranged champions are through come melee champs as a choice for Garen. If you have a melee champ as a lane partner that is already bad and your lane prolly ****ed up if you are not lucky with your lane enemy's champs. This is because while your partner still can farm, he doesnt have the advantage to really stand behind Garen and have a meatshield to protect him, neither can he harrass usually as a melee. Once engaged he will probably be at the same general place as Garen when the fight happens which is right in the middle and due to not being a monster die quite fast in front of Garen's eyes and not make it back to the tower, unlike Garen.

Last thing you want to lane with is a support char. You dont need support in any way so the only thing to support for the support char is himself which clearly sucks. No further comments needed on this, Garen doesnt lane with support, end.

Some important stuff:

Your combo is amazingly Q W E R =)

In early game hide in the brush and make anyone pulling off an aggro move regret it by whirlwinding at them. After level 2 always open up with Q and then Whirlwind. Silence anyone who tries to enter the brush and whirlwind. Take advantage of your passive and enjoy great hp regeneration. Garen doesnt need to go back, only buy items. If they are up for fighting before the minions come they will regret it so dont be scared of that.

See Summoner Spells Chapter for instant finishing move of awesomeness.

Use Q when you are moving around the map.

When you hit towers use Decisive strike right after an auto attack to maximize dps. Be aware that your sunfire cape will draw tower aggro if any enemy champion moves into range.

Now you are ready to BRING TEH JUSTICE!

I've encountered some tank Garen hate in the highest rated Garen guide and elsewhere. One argument that isnt ******** in terms of confusing the player with the champion is that he isnt played as tank in high elo games. Let me mention why this is so: Obviously Garen doesnt have viable tank skills. He cant taunt, he cant relocate, his ultimate is a single target damage skill, generally all his skills provide either survivability or damage, not CC. His one "CC" skill (silence) is awfully short and not even aoe. So Garen can neither pull agro nor initiate well. It is quite right that other champions make better tanks and therefor tank Garen is not needed.

Why then play tank Garen?

Because he has a passive that gives armor and magic resistance. He has an active that reduces damage taken. Notice the obvious synergy with HP and defensive stats? Furthermore he has a % hp regeneration built-in when he is not in a fight. More obvious synergy with HP. Those are all Garen skills that are wasted if you dont play Garen as a tank (2 of 5).

Another side of Garen is that his ultimate doesnt scale with any stat! (Make it 3 of 5 wasted Skills by now!) So before you get ***** on his judgement and "spin to win" nonsense take the time to think a minute on what Garen's skills really suggest: Soaking up damage like mad and at the same time bringing the pain and still be able to finish someone off. That is Garen. What they really dont suggest is playing him as a dps champion, there is a lot more champions better at dpsing then Garen then there is champions better at tanking then Garen.

So this is the news for you tank Garen haters: Tank Garen is the most viable Garen build on low and mid elo, the reason it is not used in high elo is only because in highly competitive gameplay you cant have 2 tanks and you cant have Garen as tank. That is all and it doesnt make tank Garen suck, it makes high elo suck because you cant be tank Garen. ;) I don't have much hate for Offtanks either, if you really despise being unkillable and generally OP so much then go ahead and add some damage, its not like i buy only tank items either, the core build along with a Mallet is plenty tank for whatever comes Garen's way.