Which champion should I get from Assassin updates?

I want to get a new champion from the assassin update or Riven. I'm not sure which one I should get though. I want to get someone that has high attack damage (or ability power) but not someone who would go down really fast when 1v1 full health trade or close combat. I don't want the champion to be super squishy. The only one from the assassins rework I have is Katarina, all the others I'm comparing who I should get and also thinking about getting Riven. Right now I can only get one champion, so i was hoping advice could be given on best champion with those requirements.