This build allows Garen to surprise people with how fast you can take out each of them 1 by 1. While still keeping a bit of defense in case of minor ganking.

Pros

You can practically destroy most of the champs in 1v1 or 1v2.
You deal practically 1k damage Crits.
High Armor Pen

Cons

Can be Squishy in some situations.
Not very resistant to disables(unless your using cleanse)
Tank champs can be frustrating still

Core Items

[/list]Infinity Edge, Black Cleaver, And Frozen Maul are core items due to they make the enemy so easy to kill once you get these.

Frozen Maul is a core item due that it gives Plenty of health which can reduce to chance of dying so easily, while Its passive lets you keep your enemies from running way.

Infinity Edge is a core item because without this you cant deal 1k damage crits so easily. That and it gives a lot of damage and Crit Chance.

Black Cleaver is definitely a core item since without it you can't eat away their armor with it while still benefiting from the cool-down reduction and damage.

Boots

Or

Depends if you fighting against AP or AD heavy teams I would prefer either of these boots to use since that Ninja Tabi and Mercury Treads Are equally useful due to Ninja Tabi's Damage reduction and Mercury Treads Tenacity.
Note: Enchants would varies on your situation although i recommend Alacrity for extra movement speed.

(Final items)

Or

Statikk shiv can be a very useful item for dishing out excessive crit hits which at the same time dealing 100 magic damage chain hit. But possibly Phantom dancer could be a better choice since it ignores unit collision and gives much more attack speed and crit even though its a few hundred more gold.

Bloodthirst can be good for sustaining and increasing damage from all the minion and champ slaying.

Ignite is used primarily to slay fleeing enemies if you cant get them in time or to make someone flee if needed.
While for Flash it allows Garen to really surprise his foes by jumping at them with it and a very useful alternative if Decisive Strike fails you for escaping

Alternatives

Exhaust Would make a good alternative instead of Ignite if you don't want your foes to escape rather letting them die slowly.
If flash isn't that useful to you then ghost might help you outrun attackers or help you deliver a final blow with that extra speed.

Although i got some of the basics done, I will try to improve or add onto this guide as soon as i find useful info on how effective it is against certain champs and from more testing.
Edit:Thru several matches it seems to do well in top and bottom, while managing to have more kills than deaths.

edit1:Removed a final item alternative. Added another spell alternative. Made pros and cons make more sense.
-Draconicflames