Darius is a soloQ counter pick. His is required to build tanky, with low AD if you wish. If you ever build mass AD you will get kited and die. He is not for a highly competitive environment but is passable through out soloQ.
Ahri is at a great competitive state, she very few lane counter picks but cannot be shutdown on a team scale. A little mana hungry and not one of the best extreme late game AP champions, but it is worked around with team composition and playstyle(ex. snowball, end games early, puts a lot of pressure on the enemy team)
I haven't play much of Zed but I have seen him numerous times. He is better than most people let on, he is an underplayed champion and requires a higher level skill cap compared to a majority of the others. If you are willing to put in the time he is a great pickup. Unfortunately his burst is rather low until he gets going with some items.