Riven is AMAZING, and she'll always be my go to champion. Her laning phase can be rocky most of the time, unless you're really good with her. Against a champion like Renekton you just simply have to outplay them and wait for your item and level power spike.
With her 3rd Q and Flash, you'll always have two forms of CC and can dive the back line every fight. The only downside to Riven is being CCed, but even then with good positioning, you can make plays. Buy a GA on her late game, and you can 1v5 easily.