A super farmed Nasus with a tanky build is the only answer. You can make him as tanky as you can with items/runes/masteries, and then his highest damage is limited only by how long you can stand to continue farming.
Nasus does win this bar none, but he's not the most viable pick against many popular tops. That won't stop you if you're a top tier farmer, have a decent jungler, and a competent mid who won't get ganked while you're farming at turret.
Darius built sufficiently tanky doesn't actually do much damage against anyone who isn't squishy. Cho has a ton of sustained damage when built tanky with Wits + E.
For general purposes I would go with Cho and Olaf. Full build Hecarim is also ridiculous, but is a dead pony if he can't get a good W in when focused. --- Spongebob quote goes here. If you're seeing this, I'm on my phone. Contacts in Profile