Yasuo being the most difficult option. Play shit like Trynd/Yi/Jax/Garen, or any of the other brainless top laners that split effectively, can 1v2 or more (important part) and the MOST important part, have a reliable escape. Jax ward hop, hop to allied minions/trynd e over walls, e away from cc/garen q away, really tanky/ yi ult away or take ghost/ etc
Yasuo can split push really well, but is also caught out really easily and most aren't capable of outplaying to the extent needed to beat something like an amumu/darius or whatever. You also take a lot longer to hit the point where you can 1v2 AND Yasuo is much stronger in team scenarios where he can come in after the initial cc/ults are down and play cleanup crew.
last point, you shouldn't be playing shit like yasuo in low elo unless you're a plat smurf lol.. but then you wouldn't be on this board sooo? XD