Garen is only good against people who initiate on him first.
That's a great strength. It means that Garen is essentially a wall in lane who can't be easily shoved out. ANYONE who tries to attack him even in a 1 v 2 gank has a huge risk of losing a lot of HP in a few seconds.
Any one of these champions and more DO NOT want to go aggressive on to Garen first. Decisive strike ensures that he stops anybody's combo provided they don't have an instant burst rotation or range.
But I also feel that's his greatest weakness. He himself can't properly go aggressive onto any of these champions. He can't force plays or fights.
Since towers exist, people will either do one of two things.
- Run away from him and continue this tactic until they outscale him.
- Run away and punish him for even trying to touch them.
People say is countered by 
. But A good Riven will always outplay a good Garen. Now I'm not Marine Revenge(he's the #1 Garen NA last time I checked) but I know the champion well enough to say that he's more easily shut down by hard cc than any other of the champions I mentioned.
Hell, I don't play Riven but I can already come up with a counter measure to beat Garen. Following the two tactics I mentioned above. Riven doesn't need to fight Garen fair. In fact, Riven can refuse to fight Garen at all. Don't believe me?
(Please correct me if I'm wrong here) http://leagueoflegends.wikia.com/wiki/Range "The difference is approximately 100 (i.e. 550 autoattack range is roughly 650 casting range)"
Taking into account of the ranges of Ki Burst (W) and Garen's auto attack Range.
Ki Burst = 125 Range + 100(because it's a spell) is 225 range. Decisive Strike = 125 Range (just an enhanced Auto Attack)
So If Garen tries to run in for a Decisive Strike, Riven can (and will) utilise the small but significant 100+ range on her Ki Burst to stop Garen in his tracks, and run Valor away. This will happen time and time again.
That was just one example of how Garen is unable to make things happen therefore not being able to advance himself. He simply doesn't have the Kit for it. Let alone the one spell to make plays.
But that's the reality of it, Garen was made to be easy and Riot intends to keep Garen that way. He'll never be chosen as a prime pick for anything than his main users because of how useless he is when it comes to his overall style of play. And with the late addition of mobility to the league, how can you legitimately expect him to truly make his own plays when everything keeps jumping out of his range?
