Before I begin, please read through the entire thing. If i make a mistake in my analysis, please feel free to point it out in the comments. This is my opinion, and I'm fine with people critiquing it, but flaming without even reading the post is immature and childish, so please don't do it.
Garen's passive doesn't really need talking about. It just allows Garen to stay in his lane for a crazy amount of time, allowing him to recover from an attack is he manages to live. He can also just farm minions non-stop early game, and HP regen items are not needed at all. This synergises with his Q, because he can just run away with the speed boost, regen health, and carry on farming.
Garen is incredibly tanky. He can tank so many hits that it's not even funny, especially when considering his damage output. He has 2048 health at level 18, and this isn't including runes or items. However, this is probably my lightest complaint in the entire post. I'm fine with tanky champions, but they need to balance his damage output and speed as well.
His Q can silence, deal truck loads of damage, speed you up and break you out of slow effects. All of this is on an 8 second cooldown without any runes or items. One of the main reasons I see when people are saying Garen isn't OP is because he's too slow to catch up to anyone, but I beg to differ. With a champ like Nasus, his W is essentially useless because his Q will both prevent him from using it and break him out of the effect anyway.
His W is also ridiculous. The passive includes minions, so you're essentially guaranteed a bonus 30 MR and Armour by the end of the game, because, if my math is correct, you only need to kill 120 minions to max the passive. That's really not that much. Not to mention that that's quite a lot of armour and mr, considering his HP count. The actual active of his W is also really strong, reducing incoming damage by 60% from both magic and physical for 0.75 seconds, which can essentially negate most ultimates (Like a Lux's ult is a perfect example, one strong and instant hit) and they're both boosted by 30% for the remainder of the time, which can be up to 6 seconds at max level.
His E isn't as bad as the other two, but it's still very strong. Most top lane champs need some kind of waveclearer, and Garen is no different. However, Garen's E shreds armor by 25% whilst still damaging them. He can also carry on refreshing the debuff if he keeps on hitting the enemy. Also, it's difficult to get away from this armor shredding without a movement ability or flash, but Garen will most likely catch up to you afterwards anyway, due to his speed buff from his Q. Garen can also cancel this at will, making it even stronger. (not to mention he deals 1% true damage to the villain, as do his basic attacks.)
Finally, his R. This is probably his most balanced ability in my book, although it is still strong. When targeting the Villain, the damage converts into True Damage, so armour and MR literally won't do anything against it. At max level, it does 525 magic (or true) damage, plus 40% of the targets missing health. This is meant to be a finisher ability, and it works well as one. However, I honestly think the missing health mechanic is unnecessary. The 525 (with no items) is strong enough, especially with the True Damage mechanic, and the 80 second cooldown without items or runes.
Thanks for reading the entire post! I welcome any criticism, because I most likely missed some fatal flaw that I didn't know about.