I find that Garren's ability to sustain is extremely over powered. His passive is great, no need to rework that. Only one good champ comes to mind when countering Garren: teemo. I'm sure I will be corrected but here is my point. Due to his health sustain and the lack of mana, he is able to dominate early game WAY to much. Perhaps his late game falls off, but with items that's not an issue. Just make him use mana, or at least scale the silence/increase cool downs at early levels. I mean after all, he does have natural tanky stats, movement increase, a natural cleanse, armor/MR buff, an amazing ult and one of the best passives. Come on now...something isn't balanced with him and you know it -_-
Edit: My argument, if it isn't clear, is that I would like to see Garren use mana. I see other sustain champs have some sort of repercussion rather it be mana cost or health cost. But none with Garren, and that is why I feel he's imbalanced.