Let's say I have a choice between 35% CDR Zilean and an extra 5 AP or 40% CDR Zilean by level 18 but -5 AP. Is the extra CDR really worth it? Is there any breakpoint at 40%? I find it a bit hard giving up an AP Q but I also don't want to buy Ionian boots or other lower damage options.
5% CDR vs 4.5 extra damage on Q before resists. CDR effects your other Cooldowns too. Would put 5% CDR over that little of AP anyday. Now what is important is whichever gives you a better early game. If you are getting that 5 AP at the start of the game it might be nicer to have that so you can pressure better early, and if you are only getting that 5% at level 18 it might not be worth it. It is all up to you. Do you want a better lane phase? Or do you think it is already good enough to where you can have a better late game?
ideally you want 45% CDR every game, it lowers cooldown on your ultimate and speed up and rewind.
As zilean you aren't the carry you are helping your carry
I main 
. Play him mid & top. I always go for 45% CDR, it's way more usefull then additional AP. Zilean really depends on CDR.
I play with 20% Scaling CDR Runes (9x Scaling glyphs, 1x Scaling Quint) & take the 5% CDR mastery. I Build 
& the last item is situational, whatever the game calls for. 
are all reasonable 6th options. Honestly the majority of games don't go to 6 items anyway.
It works great, the CDR is huge. Zil does good damage but his utility is really what makes him amazing for team fights and helping ensure your side comes out victorious.
Take cdr runes.
As many people have stated, CDR is probably the most important stat on Zilean.
Actually I consider Zilean one of the Champs you want to get 45% asap allowing you to land multiple stuns per fight. (You dont want it in laning due to mana - but as soon as TFs are about to start it's kinda important to me.)
Corret me if I'm wrong though. ^^ (Talking about Support here, btw - I guess in Mid all you want to do is blowing people out of existance with one rotation xD)
