Garen is one of the best champions of the entire game especially to play in the top lane. His passive is amazing. He can recover health when he is not taking damage from champions and monsters. Minion damage is ok. This means that Garen is able to stay longer in the lane while the adversary champion has to go to the base more often so he can level up faster and get an edge even in the early game.
As for strength, Garen has the best Q a top lane champion can have. He immediately removes any slow spells on him and gains movement speed to strike the enemy. This can also be used to go away from a bad situation or when he is on low health. Another great use for this spell is the ability to farm minions better because he can strike the normal attack and then almost immediately use the Q to last hit the minion.
His E spell is great for one thing, he can strike all the enemies nearby which is specially good to hit many minions at the same time to push the lane faster than the enemies. For example, when he has to push fast, all the minions will take damage at the same time. It’s hard to last hit using this spell so use it carefully.
His W has one thing that other champions don’t have, you gain passive armor.
In the beginning is not very useful, but in the late game is the difference between having little armor and more armor than the other enemy tank.
As for his R, you can kill an enemy ADC champion when he has about half of his health which makes the Judgement probably the best top champion ultimate. The combo is easy to make, you use Q to move near the ADC, then when he or she has less than 50% health, you just unleash the ultimate and the ADC is gone. Always save this ultimate to the ADC because it is armor sensitive. If the enemy has lots of armor, the ultimate is weaker.
Garen is a champion that has no mana take care after, he can just attack nonstop and its even easy for beginner or challenger players to play with.
What about runes and spells?
Spells first. Garen need to have the flash, the most important and core spells for all champions. The other can be ignite to give him an edge when fighting the top lane champion. Ignite does not take away a lot of damage, but is the difference between winning and losing a solo fight even when you have superior level and items.
Items for Garen?
Very easy, he needs lots of armor and HP and nothing more. Sure there are players who dominate the game with Infinity Edge and similar offensive items, but Garen shines when making a defensive build to take damage and sustain enemy offensive items in the middle lane during the late game. The most important item is the Warmog Armor because it’s super OP now. The other items take those which you can play best with. For the boots take the Ninja Tabi to sustain more damage from the top lane champion and the ADC. For the mage, just avoid him if he becomes too strong. First item? Doran’s Shield.
Garen is in my opinion the strongest champion to play in the top lane. In the late game he can sustain so much damage that he can take out a turret alone..