Single highest damage nuke...? No, that would probably still be Feast, since it's a 0.7 AP ratio, and AP is easier to come by. Or Veigar's Ult - not true damage, but ridiculously high damage.
Vlad and Kennen are no-brainers, but also Chogath (he laughs at true damage due to feast and can dish out true damage himself).
Not sure if I just had luck, but I had good success with Poppy. At least I finished laning phase with 2/0/2 and he with 0/3/0. He pulls You in? Great! AAQ, W, run around him, E into wall, AAQ again, run away. Won trade every time. His Ult bypasses Your passive, which is a pain, but You can generally kill him before You get low enough for him to finish You off since You should be winning every trade.