I'm only level 15 so far, and I've only played Nasus a few times. I main top though and I find that despite being a pretty nooby character, Garen has the best sustain by far, and his ult allows for some really rage-inducing early game kills. A tanky Garen becomes almost unkillable late game, especially if you buy a warmog. His passive stacks so well with warmog that he regens health at 50+ per second, allowing huge health regen when you're not in a fight. Plus, in addition to the black cleaver, adding a ghostblade will give you enough damage and armor penetration to hurt the other team badly with your E.
Keep in mind, I'm still pretty new. If you have any suggestions to make my garen build better, or if I said something you disagree with, please correct me. My in game name is Eatoon on the NA servers so pls add me as well.