League of Legends (LoL) Question: How do you win against Zed?

Hopesedge mentioned a few good ones. Imo Ziggs and Talon could be added to the list.

Rushing Seeker's Armguard and further on to Zhonya's Hourglass counters him completely. You'll win pre-6 easily if you DO NOT allow him to push if you aren't playing a champion who has enough wave clear to get the minions. Whenever he comes to attack a minion, throw an autoattack or an ability to get him lower each time and punish for using the W (shadow ability) by dealing damage to him or going on him if you can.

In the late game Zhonya's Hourglass/ Mercurial Scimitar counter him pretty much. Try to position yourself so that he can't get to you without taking tons of damage.

Kayle, Lissandra & Vladimir all work very well against Zed's ultimate, additionally I'd suggest building an early Seeker's Armguard to reduce his damage. In the late game you don't really want to get caught out by him as he is a fantastic duelist against most AP carries, so I'd suggest staying on the back lines of your team and try to catch him out when he attempts to go all in.

I've played many games both as zed and against him. As an ap champion you should always get an early armguard before picking up you morelloes (if this is what you're buying) if not straight up getting zhonyas first. All zed players despise this item as it seems to be made just go mess with zed players. Champions that can become invulnerable are also good zed counters. Lissandra and Vlad are very good counterpicks to zed. I also feel that being able to poke through minions is important. If zed is poking through a wave it won't do nearly as much damage. People who can poke through minions and whose kits counter zed are lissandra, urgot, kayle and probably more but i can't think of them. I would say lissandra is probably the strogest zed counterpick but urgot does well also.

Also Zeds tend to harass with their shadow. On the downside their shadow is their escape. If you can get a gank after he has used his shadow recently he shouldn't be able to escape as long as you aren't pushed.

Also Quicksilver Sash totally counters his ult.

Mid-lane assassin counters are very strong and do a great job of completely nullifying their lane opponent before they can even pick up items and rampage their team to a win.
Lissandra and Malzahar are great for shutting down burst-heavy diving assassins. Just do your best to beat your lane opponent to 6 and you should be fine.
(work best against Zed, Leblanc, and Yasuo)
Vladamere is better against assassins who can competently dive you before level 6 or champions with stealth.(like Talon)