Early game: Overall, probably Pantheon.
Late game: How late? Because the theoretical scaling of Veigar is better than any champion in the game, including Nasus. Nasus still has to get close to one-shot you with his Q. Theoretically, Veigar can get enough AP to one-shot any champion in the game except a similarly farmed Sion (who wouldn't count because he would fall prey to various HP mechanics, and also because being tanky means dick if you don't do damage).
In practical terms, Veigar is still a contender, but others might rival him.
I'm going to sort by lane:
EARLY GAME
Top: Pantheon.
Jungle: Shaco.
Mid: Maybe Ahri or LeBlanc.
Support: Braum at level 1, Blitzcrank overall.
ADC: Probably Jhin with his hot Korean build.
LATE GAME
Top: Yasuo, Fiora, or Wukong IMO.
Jungle: I'd like to say Master Yi, but he's in such a poor spot that likely isn't the case. It's probably the hot jungle Kog'maw if you have a comp for him. If not, Jax or Amumu.
Mid: Veigar or Azir maybe. Karthus is also a strong contender.
Support: Probably Sona, though that may just be her being busted. Otherwise, I'd say Taric.
ADC: Twitch.