This is my AD Zilean build that I use when I'm looking to have fun, while not sacrificing a strong game. This is the type of build that you have to try before you can make a judgement on how good it is. I am in no way suggesting this is the best build for Zilean, but it is great fun, and you will wreck the opponent every time.

The runes are self explanatory, and they really come down to your play style, so I won't say much. Health marks and armor seals are for survivability in fights so you can keep supporting and dealing damage. Cooldown reduction glyphs allow you to use your spells more, which are still incredibly useful. Armor Penetration quintessences are there to give you some more consistent damage.

The build order and final item set really depends on the game. You definitely want Manamune and Youmuu's since they help your spells as well. You might want to change boots depending on the matchup, certain items like Hexdrinker or Frozen Mallet might be useful in certain games.

I would say Flash is a must with this Zilean build. Ignite is to allow you to ensure a kill if you get low on mana. Other options would be Clarity so you can lane and fight for long periods of time, Teleport so you can defend or attack turrets, or Clairvoyance if nobody else on the team wants it.

AD Zilean is not as strong of a pusher, but he is an incredible 1v1 champion. Utilizing his heavy crowd control with Rewind can shut down an opponents movement, while his auto-attacks deal heavy sustained damage. In team fights, AD Zilean is just as useful as AP Zilean, since the only spell that really suffers is his ultimate, but it still revives with enough health to keep people in the fights. One thing to watch out for is getting in the offensive mindset, forgetting to play as a support character at the same time.