I don't play 1v1's anymore, but I usually randomed in AP just for the hell of it.
But generally it's good to get a disable + nuker.
Fayde is good with a codex from the start. Or succubus with a codex.
I prefer Succubus when I play 1v1s. Mesmerize + ult + codex + heartache = dead.
Just gotta make them concede early if they have one of the more late game heroes.