I've been playing garen since he first came out and I feel I've found a great balance with him and done fairly well with him. I've noticed that everyone has a different approach with him and my approach hasnt been mentioned yet so i'll share my method as well.
The way I see garen is not as a tank (cuz I dont believe in the term and role for this game, i'd rather break it up into initiator and controllers) but as a guerilla tactics fighter. his kit supports this fighting style very well. the idea is he'll jump into the battle full force and do quite a bit of burst damage, then run out once its done. so hes got decisive strike to jump into the battle and close the gap and does his inital damage while silencing the target, making them unable to get away for a few seconds and must take the damage. then you have judgement to deal the damage while they're stuck in range. while courage is there to help you survive the inital hit. then afterwards you have your ultimate to finish off your target if they're low enough or you run out and get out of combat to heal up with your passive. rinse and repeat till opponent is dead.
so thats why I feel he should be played this way cuz thats how his kit makes him strong. so now i'll list the type of stats im looking for. (in order of importance)
1. hp, it has great synergy with his passive and also will get extended by courage's natural resistances with the passive and active abilities of it. since his passive regens health by a % of his max health, the higher your health is, the more hp your regenerating every second and that means you'll be able to get back into battle sooner without worries.
2. crit chance, since your abilities are able to crit, you want to capitilize on it. I put crit chance runes on him so that early game my judgement will be able to do massive damage right off the back. also since Im playing him in a gurellia tactic style, I want to do as much burst damage as possible.
3. AD, all his abilities except his ultimate is AD based and have great scaling. also you want the high burst damage so that you can kill your opponent quickly.
4. CDR, cooldowns are the only cost to his abilities, the more you can lower it, the more you can use it to help farm without worrying about it being on cooldown when you need it for a battle. and the faster you can farm, the quicker you can get your items to cause more damage
5. armor/magic resistance - you want this but you dont want to go overboard. only get items that have one of the above stats so that you can gain the most out of the item such as atma's impaler, sunfire cape, Maw of Malmortius or Force of nature (though this will be removed soon) and avoid things like wit's end or frozen heart (though in some situations it could be ok, though its probably best to be taken by someone else)
Stats to Avoid completely are:
AP - for obvious reasons, he gains nothing from it
Attack speed - cuz this style is gurilla tactics, almost all your damage is coming out of your abilites and less of your auto attacks, so attack speed will not improve your damage at all.
Lifesteal - again same reason as above, your about burst damage from abilities, so theres no benefits here. your not gonna be gaining much from this stat at all.
and of coures anything that has to do with mana. cuz you aint got non
so some example builds i've done are (in order of importance, end items will be optional/situational)
boots of swiftness (of course merc boots are viable and some times prefered depending on the enemy composition)
Frozen Mallet (these 3 are my must have in every game)
Maw of Malmortius/Atma's impaler (depending on the damage type I get killed by most or both if the enemy is fairly balanced)
Force of nature/Rundin's omen ( again, depending on damage type i've been taking, and how the enemy behaves)
Sunfire cape if I need the additional health and armor is required.
youmuu's ghostblade (if I have the space)
Lord Van Damm's Pillage (if im on TT)