Yasuo is very similar to Riven. He is significantly weaker in the melee department and during his early game, but he outscales Riven, is better in team fights, and fares better against ranged champions. He is essentially Riven optimized for mid lane.
Darius is a very good lane bully for top. He snowballs easily due to his ult, and his DoTs are downright overpowered in the earlier levels. However, he lacks defensive cool downs and mechanisms for a melee brusier. If he doesn't get fed early on, he becomes easy to shut down late game. (This doesn't happen as often as compared to other champions due to his high kill potential.)
Of course, you can always pick Teemo and make the enemy laner rage when he die by mushrooms; that is an option as well.
I personally would go with Yasuo. Despite his nerfs, once you get familiar with his kit, you will be able to pull off some amazing plays. Darius is worth considering, but he fits the same roll as melee brusier as Riven.