I'm sorry, but Azir takes the cake, and also probably requires the most skill. You honestly got to position with him well, it's more than just "auto attacking". If you don't position well, use your troops correctly, even save one sometimes for an all in, you're screwed.
Azir's farming generally isn't as good as Riven's either until he gets Nashor's tooth, then he becomes god, There is a reason he is banned way more in lcs, and at worlds than Riven will ever be banned(in fact, more champions get banned over Riven cause she isn't that good when it comes to top play, and people know how to deal with her). I'm maining Riven currently, so I know.
Going back to Azir tho, once he gets Nashor's tooth, he just becomes so deadly it's crazy, especially after getting a 2nd or third item, he is able to just shred the enemy team. His damage tops Riven's by far.
He's been nerfed time and time again but still strong as fuck.
EDIT, also he is a safe pick in lane, Riven isn't safe anymore at top now and days and is best played in jungle thanks to this jugg patch.