Riven isn't tanky when she builds full AD, she simply has a shield that can block damage. Bait out her shield before you try to burst her, or play someone who can actually absorb her damage.
Darius, however, is a bruiser, not a fighter. His great base damage allows him to coast through the early game with no damage items, while being tanky enough to out trade many other top laners simply due to damage absorption. On top of that he has built in armor pen, a magic damage DoT, and a true damage execute if you use it correctly.
Are you trying to say Riven is stronger than Darius? Are you really comparing a fighter/assassin to a fighter/tank when their play styles are not even remotely similar?