AP Zilean is the best.
I could do up to 999 Magic Damage and it's crazy.
Just. Wow.
I learned that Rylai's Crystal Scepter has good synergy with Liandry's Torment, because of Liandry's passive that doubles the burn damage if the champion has impaired movement.
You don't really need Rylai's on Zilean though. He has a stun built into his Q>W>Q and his E slows far more. Rylai's is for people that don't have it built into their kit and can put out more consistent attacks to keep the slow on. You'd probably be better off with most any other AP items than those two on Zilean, imo.
Zilean has no damaging abilities except his instant double Q or just one Q. But that deals ridiculous damage
He should be changed from Mage - Support to Support - Assassin
cus you have a gap closer / stick CC (speed buff/slow), self peel after diving (ult) and instant burst damage (double Q).
Doesn't that sound like a specific someone 
Insane Sick Damage Combo with lichbane and Nashor's E self Auto enemy carry Q enemy carry Auto enemy carry W Auto enemy carry E enemy carry or self Auto enemy carry Q enemy carry Auto enemy carry R self Auto enemy carry
570% AP Magic Damage + 1.5 second stun + 450% base AD Magic Damage + 240 - 690 Magic damage + 600% AD Physical Damage
40-99 % movement speed buff to self or slow to enemy 1100 + 200% AP resurrection.
Include masteries for even more damage
GG.
In a perfect world sure, but usually you get bursted down by the second auto. Zilean can be fun to play as a mage, however you'll quickly see that he gets frustratingly easy to beat.
